Kazan: LOL! Get a grip, it doesn't have to be done RIGHT THIS SECOND. There's always tomorrow, no pun intended. I swear, its gotta be possible SOMEDAY. Physics engine? Nah, its just a matter of making people see each other differently. You see them fast, they see you slow, or vice-versa, and neither of you see anything wrong with your own ships.
Theoretically, find a way to make time compression work for SPECIFIC objects with an inverse compression/dilation relationship like this, and BOOM, its all good. No physics engine modification required, although it'd be best to stick with only 1.5-2x time modification fields as anything larger would be way too much

Um yes, firing primaries at objects caught in the fields from outside the fields WOULD be weird. You might have to use missiles, but if the enemy is in an *accelerated* timefield, you're in big trouble, because they will be able to run away long before the missile even hits the timefield

Keep in mind that even though relative to "real" time Alpha 1 is moving 30 mps, his speed on his HUD is 60mps and the Sathanas thinks its 60 as well because that's what his speed really is --- its just that the targeting computers can't understand its 60 meters per TWO seconds, not ONE.
However, it doesn't matter. A shot from the Sath hits the timefield aiming for a 60mps speed, slows to 30mps, and hits Alpha 1 dead on anyway.
Long story short you do NOT want to be in a decelerated timefield. You'll be a sitting duck.
Besides, it would be fun to have a campaign where the fabric of reality itself starts getting twisted around, perhaps by the Shivans.
I have always enjoyed sci-fi shows where relative time starts getting screwed up.