Oh, I'm sorry to hear the barrel is suspect...but also glad that you've taken the initiative to examine yours. GI Joe had a saying about that.
https://www.youtube.com/watch?v=R0eD5ABK0wc
As I mentioned above, I have 3 SPA rifles and all of them had rough machining typical of budget guns. Fortunately they grouped great after some basic deburring and smoothing so that suggested to me that SPA is doing fine in terms of the bore itself. Most of the stuff I've read on the forums agreed but it sounds like the barrel on yours is questionable at least. On an unchoked barrel like this, it certainly has more potential to negatively affect things than if it were choked. What I would do is push a few different pellets through and pay careful attention to how much force it takes to push them through the first portion of the barrel compared to near the muzzle, particularly the last inch or so. If it's obviously moving like a greased pig before it emerges into the world, it's a problem. Might want to contact the vendor and see about a replacement but if you have the means to fit an aftermarket barrel, it definitely seems to be a platform worthy of the effort.
For the breech O-ring, I replaced the spongy metric OEM O-ring with the nearest fit "inch" (AS-568) in 70 durometer Buna-N. According to my notes I used a -012. The OD is about 0.030" larger. I remember it was a bit finicky massaging it into place but I managed to do so without removing the block. It hasn't tried to pop out and most importantly it cured the puff of air that I had been getting.
Regarding stability of the shroud and barrel, I initially used a 1pc mount to lock the receiver and shroud together. However I've been using a 2pc mount for a few months now and the POI and groups have been very stable, even with mild bumps of the muzzle against my shooting table. How tight you can get the barrel nut with a spanner wrench would be the biggest factor. With sufficient torque, it puts the barrel under tension which is a good thing. So overall I don't have reason to think it's sensitive but I agree it would be a good idea to use a 1pc to erase that as a potential issue. Otherwise you'll always be wondering.