Yes, that Pic Rail is there for an arm brace, waiting for someone to legally SBR it, or just do nothing with it; by itself it's OK. It's what CAN be bolted on it that can get folks in trouble here in the USA.
One stamp for 2 items? No. You'd need one stamp for the can and another for the SBR. $400 in 2 tax stamps. One to allow the SBR to be built by you by putting a stock on it. Another tax stamp to transfer the suppressor to you. But an arm brace is currently OK with no tax stamp. But a brace and a stock while looking similar are different. The manufacturers and retailers should use the correct terms in their text keeping you out of trouble. And yes airsoft is fine for .22 rimfire and will save you $. Just make sure it's not a stock and don't think it's mil spec and ready for the Ukraine. I used an airsoft folding buffer tube for years on a Charger that I used in steel challenge and the only reason I no longer do is because as a SBR I can put anything on it and I just changed it out to better stock. No fault of the air soft part. To use it in competition I had to SBR it since an arm braced handgun is against the rules. But as a short barreled RIFLE, a rifle is legal. Same firearm, the difference is one is a handgun that won't fit in the box and the other is a rifle with totally different competition rules.
Just go onine and do a 'net search for "folding arm brace". I'd be surprised if you don't get a days worth of hits to open up. When I bought mine I did that and price shopped for the one I wanted. You'll find the "same" models at times, one made of aluminum for big bux, and the "same" model made of plastic*. The plastic is fine and generally much less expensive.
*I don't mean 3D printed.
Cheek weld... Again the 'net is your friend. Go to youtube and type in "cheeking handgun" in it's search function.
If you don't know how to do any of that get back to me.