I recently purchased a SW5 from the used rack at a LGS. Wasnt really looking for one but had been considering building a 9mm AR for fun and this fell into my lap instead.

Is there a way to tell if a SW5 was factory built or sold as a receiver?

The paddle mag release seems to be broken/floats around in the housing and the psuh button mag release seems to be hanging up on something when pressed. The rifle didnt come with a mag at the time of purchase so I cant verify it doesnt work correctly yet.  

It has a MP5 length barrel with a fake suppressor attached, it appears there are threads and a lock washer onthe barrel at the rear of the fake suppressor. Did SW use, or was there another company making,  threaded barrels instead of 3lug barrels for their fake suppressor carbine builds?

I want to remove the fake suppressor and have a larger shroud attached to bring it to 16" and be able to attach a real suppressor inside the shroud. Having a threaded barrel would simplify that quite a bit.

I don't think any "company" built stuff of SW5 receivers, maybe individual mp5 builders did though. Dunno on the mag release issue, there is a spring in there and if its bound it could explain why nothing is moving and the paddle is flappy. Is it an actual paddle mag release or one of the tac-latch dealies? No idea on a threaded bbl, I though they were all 3 lug.

My understanding is that SW didn't see receivers and you probably have a factory built gun.  SW and Coharie, and
Todd Bailey have a pretty spotty reputation online so hopefully yours functions as it should.  When I was looking at these (ended up buying a Bobcat BW-5 10 years ago) there were many people that had issues with these and the parent company, so a broken or defective mag release wouldn't shock me.  Not trying to rain on the parade...

http://www.ar15.com/archive/topic.html?b=1&f=5&t=433692
https://www.ar15.com/archive/topic.html?b=6&f=9&t=291261
http://www.arizonaresponsesystems.com/notes/notes-hk/hk-review-sw5/page-hk-review-sw5.shtml

I finally got a chance to shoot it, runs fine, mags lock in and the paddle mag release works as is. The kids all loved shooting it.