Heavy weight reduces recoil when it's balanced correctly, but when it's not (and using a heavy weapon meant to be brace fired primarily or two handed fired when not able to brace on handed is going to be one of these cases) it just compounds the recoil. If you doubt that, take your rifle and fire off a few shots one handed, then strap a ten pound weight to it and try it again.
Yes, when braced most MGs are don't need two hands supporting them because they are braced. The whole point of the brace is to force the weight of the gun into the brace and let the brace absorb the recoil.
The lines between machine pistols and submachine guns blur as do those between smgs and carbines. Most carbines are just short barreled versions of an assault rifle, but some, especially bullpup models, are only available as carbines and roughly the same size as an smg. It really comes down to the definitions you use. One of the most common distinctions is that most people define SMGs as using pistol ammunition and carbines as using rifle ammunition.
None of this really changes a fact that has been completely ignored here though. All this talk about SMGs being two handed weapons makes an assumption that Pistols and Machine Pistols aren't two handed weapons, but in real life nearly every stance taught involves the use of both hands on them just as much as using both hands on an SMG. OMG what can this mean? Guns are more accurate with two hands...mmkay. Universal standard here, no exceptions.
Of course with all your first hand knowledge you should probably know that. Of course, SR4A is an action movie, and unless I've missed the headlines of a real life Live Action Hero, you probably don't have any first hand knowledge from the shadowrun universe which is what the books discuss. Real Life is ---> that way. Gritty Realistic Combat in an RPG is <--- that way. This is shadowrun, home to campy humor and utterly unrealistic damages since 4th edition. Yes, book knowledge of shadowrun over-rules first hand knowledge of non-SR weapons in SR.