Hmm... I’m still in the armchair general phase at the moment.
It’s a shame that spells cannot inflict or remove statuses – seems like a fairly limiting omission from the design. Ideally, it would be nice to have status effects – Poisoned, Diseased, Dazed, Bleeding (different type of continual damage), Afraid, Horrified etc. Then you could have spells which are countering specific statuses (i.e. "Armor of Faith" helps with Afraid\Horrified, "Cure Ills" helps with Poisoned\Diseased, but maybe not Bleeding).
It would also prevent the situation where currently there’s nothing stopping two separate poison attacks both inflicting continual damage on the same target – you may want to limit the amount of damage poison can do per turn, and the way to do so is by using the Poisoned status.
As for making a spell that increases poison resistance after the fact – that would be something to try (not sure if the roll to check for unit’s poison resistance is done each turn, though). However, what should its duration be? If it does not match the remaining duration of the poison exactly (and it won’t, generally), then either the poison will kick in at a later phase, or else the unit will have poison resistance for a few more turns... Hmm... maybe it’s a feature – Neutralize Poison removes all poison, and keeps you from being poisoned for three turns.