I have some experience with molding a rear bumper on the car, as I just added a duraflex Nismo a couple months ago..
Right below your finger in the third photo, there is a bracket beneath the body. Its attached to the frame of the car. It has 5 clips that were made for OEM bumpers to clip into and hold them into place right below the taillight.
When I got my duraflex bumper, there was extra paneling near that bracket, but no HOLES or INSERTS for the bumper to clip into. So i had to custom drill holes for the bracket to connect to... It seems to me that may be your issue. The sagging or not filling out looks like its from not being properly fitted onto that bracket/mount on the underside of the bumper, I could be wrong though.
My rear bumper was off fitment by around 8-10 inches. Took 2 hrs of heat gun and custom molding to make it fit and look perfect..
Goodluck and hopefully its fixable
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2009 Brilliant Silver BASE 7AT | TEIN S-Tech Springs | K&N Drop Ins & z1 Maf Tubes | AAM S-Line Short Tails | 5% Tint | 20MM Spacers | Sport Spoiler NickTurnon's 370z Build
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