A lot of questions, but here goes. The pistol was accepted by the military and has the WaffenAmt (Army Weapons Office) acceptance stamp. There is no way to know who it was issued to or who used it or, if anyone did, what the circumstances might have been.
Barring the presence of some marks you have not shown, the gun appears to be a regular Walther PP of the mid to late WWII period. It appears to be in good condition, and I would say its value is around $600-700. Obviously, the value will increase over time, but I wouldn't expect any dramatic increase in the near term.
Unless there are "capture papers" or a family history, there is also no way to know who brought it to this country or where it has been.
Without actually looking at the gun, I would have no idea why it hangs up. There might be a problem with the magazine or even with the ammunition as some bullet shapes will activate the slide stop. I will note that those guns were made for ammunition using full metal jacket bullets, not some of the current hollow point bullets. Sometimes tracking down that kind of problem can be very time consuming and frustrating, and I expect the gunsmiths you consulted just didn't feel like bothering with it since they could not charge enough to cover their time.
Original WWII Walther grips are sometimes found at gun shows, but are scarce. Both post-war grips and reproduction grips are available. Further, the PP is still in production in the U.S. and current grips should fit. Gun Parts Corp. (www.gunpartscorp.com
) has post war PP grips for around $45.