Kinda a simple question, but why is it there are certain mage spells that won't carry over through logout? Seems like it's an OOC function that affects the IC character's gameplay (primarily only in the situation of having the bad luck to log in right where an enemy happens to be standing, but why's that chance there in the first place?)

If you cast shields on someone other then yourself and they become immune to damage done to them it drains a little mana from you each time. I imagine that if the code started trying to drain the mana of someone not logged on it could cause some wackiness hence why when an invoker quits all the shields he has granted go away. i suspect that it also removes their own shields just cause the code doesn't distinguish between them being on the caster vs any ol random person.

Simple solution to your worries is to quit in less then obvious spots. Places where people might not be or look for those 2 seconds you need to c teleport and reshield.