This part I am not sure of, but I think you cannot have pre-ban features on a post-ban SBR-weapon either, even if you can have a short barrel. All you can have is a short barrel.
You can't have a postban SBR with preban features, even though it is now on the NFA registry. You can, OTOH, buy a preban rifle and retain the features if it is converted to SBR status. The same does not hold true for pistol to SBR. For example, coverting an SP89 to an SBR (adding the PDW stock) would require the stock to be fixed, as you have just manufactured a new rifle.
I will not attempt the other question because I do not want to get anyone into legal trouble. The only way you are going to get a correct answer is to mail the ATF Tech branch and get it in writing. If you are truly serious about this project, you probably won't mind the short time it takes for them to respond.