[Spike]

I don't believe there's an aftermarket gearbox shell that lets you use the forward assist. I could be wrong, however. But most G&G AR platforms that I've seen have that feature built in if they're the blowback series.
One or two last shots in semi auto with a 2S lipo should be fine, especially if you're keeping the stock motor.

You may have to trim the back of the cylinder to fit in the blowback gearbox, just as a heads up.

If you want to give it a reliability tuneup, I would say shim the gearbox (especially the pinion gear to the bevel gear), install a basic mosfet to protect your trigger contacts against 3S lipo use, and install a sorbothane pad/correct the angle of engagement. The stock pistons are quite good so you shouldn't have to worry about it for some time, especially if you do your pre-emptive tune up on it.

Because those gearbox shells aren't as strong as they should be, I would recommend downgrading to an M110 spring.