This is still going on?
Here's a new one for you...
Lets say that if a battle isn't finished by turn (whatever) that it is considered drawn, and that the forces will show up again *next turn* to battle again. All afflictions/conditions can be kept (or not) and all movement *out* of that province is disallowed, while movement in (consider it reinforcements) is allowed. That might be a pain to code, but as you can already have castle seiges (this is just a siege with forced combat...) some kind of code for two nations in the same province exists (of course what happens with more than 2 nations in the province... that's tricky...).
Anyway, another suggestion to the percieved problem (which I personally don't see as a problem, but well I'm trying to be helpful
)