There are a number of spells that need to be looked into further, which seem to behave differently from the norm (making my algo look shaky at times, but they look like the exception, rather than the rule).
The standard behavior for most status inducing spells involve the following:
1) If the spell fails at the hit rate level, it fails altogether... regardless of targeting.
2) There is a status priority to follow.. like you can't inflict Charm status to a target that's already in Toad status.
However, as I did a little digging, there are spells that are the exception.
Toad/Piggy/Mini - FF4A Specific
In FF4, status priorities took precedent.. you couldn't inflict Mini or Piggy status on targets that had Toad status.
In FF4A, this behavior is ignored as you can inflict all 3 statuses in any order you wish.
Toad/Piggy/Mini - any version
I was doing some Toad testing with the ToadLady/Bog Witch. It's peculiar in FF4 that she has a very high magic stat.. good enough to hit 100% of the time.. but it doesn't quite do that. Toad status removes any semblence of magic evasion.. but Toad relies on its hit rate to succeed! So, I figure there's a behavioral difference...
Status spells seem to behave differently by who the caster is... monster or character.
For a character, success primarily runs through the spell hit rate plus associated bonuses via Wisdom or Will Power. It doesn't matter if it is single or multi-targeted.
For a monster, single targeted status spells are generally a non-issue.. it follows the standard magic algo. For multi-targeted status spells.. the formula seems to operate more like this...
Spell Hit Rate = Base Spell Hit Rate + (Spell stat/2)/# of targets (* Slot 1 bonus, if applicable)
This would explain the ToadLady/Bog Witch better.. including Zemus's Mind.
The ToadLady's success is tested against each target... probably against a magic defense multiplier of 1... (since it has the status multiplier bit set)..
Her success looks like this..
Spell Hit Rate = 50 + (223/2)/5 * 5/4 = 90% hit rate - the number should be lower... hmmm
For Zemus's Mind (for multitargeted Charm)..
Spell Hit Rate = 65 + (99/2))/5 * 5/4 = 92% hit rate
Of course, this needs to be looked into further... hence the thread title.
Sleep - All Versions
Other than the major difference between the SNES and GBA versions is the level factor for the Slumber Sword... occasionally this spell doesn't hit all the targets... I wonder how that level check is applied... You can occasionally see this spell hit all targets but 1 late game.. whereas early game, the spell doesn't quite hit every target.
Stone/Break - FF4A only
This seems to exhibit the same issue Sleep does in that aspect that it doesn't always hit all the targets, even when the spell succeeds. I doubt there's a level check for this spell in the first place.
I should get around into taking pictures of Sleep and Stone/Break.. but it's rather curious it hasn't been looked into further.

Fixed a few descriptions.