How is the recoil and flash on at 16" barrel?
Recoil is a bit more stout than the full length bbl, flash will also be more noticeable. Neither is detrimental IMO
Buttstock is non-humpback. Is that any good, or would it need replacing?
The buttstock should be fine if you are ditching the scope and going back to the iron sights. If you do mount another optic, the added height will mean a chin instead of cheek weld to align the eye. This would be similar to an M1A with original stock mounting optics. Again, not really detrimental once you get used to it.
The stubby FALs are less reliable than full length or even 18" versions, but as most FALS are as reliable as anvils, the bit of drop may not be an issue. If it runs fine now, I have no reason to believe it won't continue to put rounds down range with boring regularity.
I'd make the trade, but then I'm more of a rifle guy than a handgun guy anyway. Afterall, a handgun is just a means of fighting your way to a real