It's actually fairly easy. Any spell that is single target (ie Hyperdrive, Fire3, etc) can be Runic'ed. Any spell that is party-wide (ie Ultima, Meteor) cannot be Runic'ed. I don't remember if there are exceptions to this or not, been too long. 
You can Runic Ultima you goof.
There is no hard and fast rule of thumb. You know Runic fails when there isn't a reaction to the spell attack.
My answer for question D: There isn't any other reason, but consider that Vanish itself removes its own status or the possibility that a Vanished Celes/Gogo under Runic has this status removed is the logical reasoning for this behavior. It's very much like how Imp temporarily makes Celes/Gogo an Imp when Runic is being active... these are cosmetic effects that would probably need some recoding to handle certain cases.. it's probably easier to fix them after the spell effects.