Third, if a running Focus Fusion reactor (5-20MW range) is breached (by, say, an anti-tank kinetic penetrator), what would happen? Explosion? Just some toxic vapor?
No explosion. Yes, there would be toxic vapor although not much. Radioactive methane–disperse very rapidly and very short half life.
Fourth, how practical would it be to shoehorn a Focus Fusion generator into something the size of an M1 Abrams Main Battle Tank?
Anything that can power a truck can power a tank. Anything that can power a ship can power a warship. It would fit in anything truck-size or large, not a car.
However, as a weapon itself, we say the only way it could be used is to drop it on someone. 3 tons of cement would be much cheaper.