While it is possible that your new RTG bolt was responsible for the shrinkage of bolt gap during break in, it would be fairly unusual to go from 0.014" to 0.008" in 100 rounds of use, especially after the gun has already seen 300 rounds prior to that. I own one of these bolts, and it has been a solid performer. If there were problems with them, it would have been reported on this site at some point, and I haven't seen anything posted.
If your new HK locking piece doesn't stabilize your bolt gap, you probably have a soft trunnion or your barrel may not be pinned properly. Either problem would hopefully be fixed by the manufacturer, although I'm personally unfamiliar with PTR's warranty and customer service. A complete loss of bolt gap could potentially be a safety issue, so I would think they would want to make sure that a PTR customer is safe even if you aren't the original owner.
By the way, welcome to the site!