Replace
I believe you need to send that in and have it replaced, glad that was the gun and not some part of you, sadly we humans aren't as easy/simple to replace. Sorry if this is out-of-line because I'm just basically reiterating what others have said. But I really like the USP which is what made me click on this to begin with, I won't go into detail or anything like that, but its the only HK made pistol I'd carry everyday. I'm glad others think the same way. Send that broken beauty in and have it replaced.
Gunsmiths can do amazing things, I don't need to elaborate. But I wouldn't feel 100% confident 100% of the time packing that USP even if a smith was able to satisfy you to the point you believed it was just as before. That might sound awkward but I'm an awkward person so if you don't understand what I said just glance over it.
One last thing, I would like to add an additional reason that I believe it should be replaced and not fixed as best as possible via a smith. I do not know if someone has stated this before, I haven't read every entry so sorry if this is just repeating something, I'm new here and this is my first post anyway. But a great person in the Firearms world said that "A pistol is very different than a rifle in many ways, one of those ways is that a pistol must function perfectly every single time it operates". I believe that too. With that being stated I thus believe that is only one more reason that you should have the pistol replaced as seen fit by the manufacturer. I wouldn't allow my life and the lives of the ones I love depend on something that was fixed instead of replaced in this case. Obviously all situations are different, but that is how I feel with this situation and why I believe you should take the action I suggested. Sorry for the loss of your bike by the way, and my father rides a BMW F650 and carries his in the same position, I say that because others have noted that is an awkward or even that its a bad way to carry. He's never had any problems. Anyway I hope everything turns out alright I'll try to keep track of this so give an update when you can if you want to.