Yes. It was PS+ free game when it originally released. Brilliant game. It was the PS release that gave it the popularity. I would rather it on Xbox so maybe it'll be game with gold soon.
If you're only putting it on briefly for mot then I'd think the car will be fine. One issue you may have is if it was mapped for the decat the fuelling will have changed and fitting a cat may still fail emissions.
Sounds like a good low mileage example. If the cambelt hasn't been done due to the low mileage I'd put that on a list of bits to sort first. GX are pretty sound.
If the overflow is being forced out it's being pressurised. If it is head gasket most common way for Subaru to go seems to be exhaust gases entering cooling system. Did you have it sniff tested?