I have not gotten around to figuring it out yet.
The Immune byte is simple... when properly combined with elemental properties, it would reduce damage by that elemental (magic or physical) to 1. This is most notable with DK Cecil's weaponry vs Zombies.
The Adamant Armor has it (not quite sure in the PSP port, and IIRC they revamped TAY's version). However, it was limited that it was specific to elemental magic, not maxHP/currentHP based elemental monster magic where it would just follow the resistance rules for those formulas.
The Immune byte doesn't work like Absorb, where it attaches itself to resistance... it must be "built-in" to the armor itself like the Adamant Armor. The Glass Mask had this bit in the original... and it was removed in the WSC/GBA versions.
A side effect of the Immune bit was permanent elemental weakness in the original game... so as soon as you removed the Adamant Armor, you would be weak vs Lightning and twice as weak vs Fire and Ice (because of the whole Fire/Ice armor resistance/weakness behavior). It also had the side effect of providing 99% evasion (not magic evasion). I suspect it was intended for magic evasion, but since this feature was unfinished and revamped in later ports (like the absorb property), it kinda got lost.
Anyways, Makeshift Cannon sucks IMO unless you have multiple targets involved. The biggest problem with the Band is that you consume a lot of MP (58) to dish 10k or so damage, but it's really a multi-target based attack, so wasting it on a boss is just that... a waste. This is especially true when you can easily add damage through Holy or Flare from Palom and Porom.
Monster targeting tendencies change when you goto a 2-3 or 3-2 formation (using football terminology). The first number denotes the characters in front row and the second # denotes the # of characters in the back.
Twincast is literally the Black Magic Band from FuSoYa and Golbez with I think the actual chance of casting Meteor. The only difference is that it consumes more magic than the original band it is based on (Twincast is 40MP, whereas Black Magic Band is 30MP).
Delta Evolution is really overkill late game (Ceodore+Luca+Ursula), where the limit cap is reached pretty quickly. It might be a good idea to have the Limit Ring equipped, but it's easier to rely on the cheaper version of it Twin Rush (Ceodore+Ursula) where the MP pool isn't limited to Luca (in the Makeshift Cannon party, Luca is the one that has the least amount of MP).
In finding out how bad Delta Evolution, I've also found that Healing Wind (Porom+Tsukinowa) is insanely powerful. It's twice as powerful as Curaja (seriously, although this was at a moon phase that increased White Magic) and adds what looks like Esuna to the party. 40MP for two characters is really double the power.
I've subbed Luca out of my current party, which now has Ceodore, Ursula, Palom, Porom, and Tsuiknowa. I'll play around with the Rapid Fire Band (Ceodore, Palom, and Tsuiknowa, 20MP) and see if this combo looks promising.

Rapid Fire Band is good... low MP cost (20), but its potential can be unleashed with a target weak vs Fire and with the Limit Ring. However, it can be effective long term vs monsters that resist Fire as the damage dealt is stronger...
So Ceodore, Ursula, Palom, Porom, and Tsuiknowa is not too bad.
I'm onto Cecil, Palom, Porom, Kain, and Rosa. I want to look into Break Breaker Band (35MP) and see how good this is... will look into Team Baron for better insight later.
Some particular bands I want to look into would be Sky Grinder (wondering if Double Jump with Ceodore+Kain is better). Porom and Rosa have a particular non-healing band (Holy Ray, 50MP) that hits every monster (think Double Holy before actually learning it). Then there's also particular bands with Cecil, Kain, Rosa (Trinity Crusade for 35MP and Effect Form for 20MP).
Substituting Ceodore in this group (to remove Palom or Porom, Palom would be a likely candidate actually since there are a few limited bands with him involved whereas Porom has a few), probably increase in the Holy based attacks... with some healing bands with Rosa.
I originally liked the idea of Cecil, Palom, Porom, Lenora, and Luca if only to make Cecil the designated healer for Palom for Ice Break... but having Ursula is better for Petal Whirlwind and Chakra does the same thing for free.
I had thought of Cecil, Golbez, Ceodore, with Palom and Porom, but that would just add only Golbez's limited bands, where it isn't that the most cohesive (well, specifically Golbez anyways).
Cecil, Palom, Porom, and Rydia has some potential.. with probably the third character would be Rosa... unfortunately this is already a mage heavy party. This would allow some special bands (Cecil+Rydia for Iron Flash, Rydia+Rosa for Holy Burst, Rydia+Palom for Thunderstorm, Porom+Rosa for Holy Ray).
Twincast's other spell is "Double Flare" as noted by the sound effects. It's technically worth the 40MP, but this hits the damage cap easily.
Glass Mask = worthless. Even the Thornlet in FF6 was more useful as a trade option...
If only the Gigant Axe was like, useful vs Giants... because it technically loses out to Thor's Hammer minus the elemental aspect (Thunder).
Thor's Hammer provides +10 STR, but also casts Blitz, so it probably uses the same value it did in FF4A (GBA port) for the spell multiplier (it's either 7 or 8). Finally, something that can kill black flan instantly.
I will look into combos dealing with Tri-Disaster (Palom, Rydia, and Leonora) and Wild Whip (Rydia, Izayoi, and Harley) soon.

Some updated list (in my playthrough) on physical based bands that ignore row:
Cross Slash Prime
Rapid Fire (I think)
Vibra Plus
Holy Cross (I think)
Sky Grinder
Spiral Blow
Trinity Crusade
Vibra Prime
Some bands that depend on row:
Rocket Launcher (it actually depends on Cid's row, not Ceodore's).
Machine Break (it actually depends on Cid's row, not Cecil's... not Limit Ring worthy) - damage is based on a combo of Cid+Cecil with a 25% damage bonus, and racial bonus applied when applicable.
One of the bad things about Team Baron is that it really lacks a black magic user... so it's very dependent on the multitargeted bands. It's easier to sub in Golbez or Rydia for black magic and some better alternative bands. Team Lunarian (Cecil, Ceodore, Golbez) with Kain and Rosa (or Team Young Baron plus Golbez) or Team Baron Mist (Cecil, Ceodore, Kain, Rosa, Rydia) is definitely more optimal. "Baron Mist" is in reference to the Mist event... remember that Cid wasn't really involved in any of that and remember that part of Ceodore and Kain's journey involves Mist.
Many of the late game bands allow Ceodore to be in the back row, which allows him to not lose power in his bands.
Trinity Crusade does 15k damage and is the actual equivalent of Kain (Jump), Cecil (Fight), and Rosa (Holy). This is far better than Vibra Prime where it is more geared towards multiple enemies, and arguably be better than Makeshift Cannon for the damage that is dealt. Whatever you construct with Kain, Cecil, and Rosa is pretty damn good...
Effect Form is disappointing... as it is specific to the three characters involved with the Band. However, the MP cost for it can be justified (Curaja being 40MP, Protect is 9MP, Shell is 10MP).
Sky Grinder is very expensive for what you are getting out of it... (45MP). Trinity Crusade is better (35MP). Spiral Blow is close enough (22MP), but I think that has a limit...
Divine Heal in some ways looks like Cura, but it's really a combined every of Curaga from both Rosa and Ceodore. It is certainly affected by the phases of the moon. IIRC, in a favorable moon phase, it will also do Esuna or something (it seemed to work vs the Duke Malboro's paralysis attack).

Divine Heal was noted to be affected by the phases in a Walkthrough, so I'm not imagining things... :P
I assembled Team Girl Power (Rosa, Porom, Leonora, Ursula, Luca) thinking to use Petal Whirlwind... however I had a simple backup plan for Omega.
One of the bands I wanted to look into is Pure White Magic (Rosa, Leonora, Porom). It costs 99MP... so I wanted to look into the details...
It casts (MP consumed normally in parenthesis):
Curaga (not Curaja) * 3 (the combined power of the three white mages) = 54MP
Blink to all characters = 40MP for 5 characters, only 24MP for the 3 required characters in the band
Haste to all characters = 125MP for 5 characters, 75MP for the bare minimum
Float to all characters = 8MP
So in order to have done this band all in a separate fashion, you would have consumed 227MP total... despite needing 297MP combined (99MP * 3 characters).
I'm surprised they didn't throw in Protect and Shell in the deal...
In any case, this was well worth it for Omega...
For Omega, all I did swap in was the Thor's Hammer and Lightning Claw. I put an Adamant Shield on Ursula for Charm resistance specifically... so basically the bulk of the damage was dealt by Ursula and Luca. The three white mages in the back row would heal or get rid of Omeguard as necessary (it's not like they hurt). I lost the first battle because I forgot to use Spider Silk... especially since the annoying beam attack is specifically 1/2 maxHP damage + Sap, which means that 2 consecutive hits is instant death (which means Curaga/Curaja healing is required immediately - Curaga is basically all you need). Note that Curaja IIRC takes more time to cast than Curaga (this might have changed in TAY, but this was standard behavior in FF4). Ursula would use Tenketsu or Attack (Tenketsu preferably) while Luca would use Big Throw or Attack, depending on the phase of the moon. This technique should simply work across the board as the requirements outside of good armor and good resistances will help out. It shouldn't need all the overpowered stuff like Crystal Rings and multiple Ribbons (or Pink Armor for that matter)... just Headbands (Confuse resistance), White/Luminous Robe (SPR Boost), Protect Rings (should have 2 at this point), Dragon Armor set (elemental resistance for Luca), Red Jacket/Red Cap (Fire), Cat Hood Ears (Ice), Mystic Veil (for SPR and INT boosts, 2 max) is all you really need. Having to juggle them is another matter altogether...
I didn't see Omeguard in action though, so I assume it was some reaction to magic attacks or something along those lines... I didn't see its "Arm" (damage+confuse) attack either. Oh well.
In any case.. the monster was
harmless.
Hero Shield works exactly like it did in FF4A (GBA), outside of the modified stats (+5 all stats, 55% evasion). The bonus is the "Absorb" attribute, but it's not that useful on Cid or Luca because of their hammers... but great for the important 4.
The Lunar Mail I got was interesting to say the least. Another "vague" resistance set... which seems to include the following through testing: Paralysis, Confuse, Death, Break... all of the nasty stuff you'd find in the Depths. I read in a walkthrough that this was worse than the Crystal Mail? Have you actually seen the resistance list that the Crystal Mail provides? It's not even a half Ribbon replacement! The Lunar Mail is that and then some... (+5 STA bonus is meh).
Petal Whirlwind's description is actually in reference to Tenketsu, thus not actually referring to elemental weakness. This was tested vs the Duke Malboro (it has a Fire weakness) and was disappointed (no chance). Still, it's not a bad Band, but it's not as good compared to other better bands like Trinity Crusade.