Are the 10rd. Factory mags augmented in such a way that they can not be made to hold 12 rds? I am not trying to get into legal trouble either, so that is not the nature of the question.

Also can the USP mags be used in the tactical models? Or do the Tactical Models take the Socom mags?

Quoted: Also can the USP mags be used in the tactical models? Or do the Tactical Models take the Socom mags?

The tatical model is still a USP.  Full size UPS mags and USP tactical  mags are the same.

The Tactical mag that comes with the gun and the ones that are sold specifically for the Tactical are only different in that they have the extended base plate with the finger rest.  You can swap those into "regular" USP mags if you like them.  The frame of the Tactical and USP is identical since the Tactical is just a USP with the extended oringed barrel, high adjustable sights, and a match trigger installed. Oh, and the slide has the super cool word, "Tactical" engraved on it.

There is no way to make the 10 round mags into 12 rounders.  The way the baseplate works prevents that.  It's a two piece system that takes up too much room inside, and the inner piece is what pushes the clips out into the body of the mag.

Well I guess that answers that......

Why in the heck are the HK mags so dang expensive?? I mean I know the pistols rock and all, but man $50 a mag, that is just ludicrous!

Yeah, tell me about it.  I paid that much for a total of 4 mags.  During the ban.  So now I have a total of 6 10 round mags, that would only cost me half that now.  I looked at them with the idea of modifying to 12 rounds, but couldn't see any way to do so, and I think the stupid ban mandated that there was no way to do it.
If/when I need to replace these, then I'll do it with the 12 rounders...maybe.  I don't know if an extra $25 is worth it for just two more rounds.  I mean it would be the one time I'd need to reload, but that's pretty unlikely, so I'll just take that chance.

Oh, don't be tempted to try out those Pro-Mags.  The things are junk.  Maybe for range use only where you don't want to drop good mags into the dirt, and for some practice on failure drills.