It's probably ABS which I think you can find some specialty epoxy or adhesives to do it but, probably not in place and it wouldn't be pretty. The only other thing that comes to mind would be a gusset of aluminum or stainless (patch) to reinforce and pop rivet it in. It will either work or you can guess the alternative. If it were me, I'd look for the new pan that fit and and foam it in with Great Stuff since you know where the weak spots showed up. My shower ( looks like yours)had significant flex around the drain area floor, which I took care of last year, knock on wood, been good so far. Drain setup kind of looked like the photos in previous post on the thread. Just my $ .02 worth. Good luck, clue us in how you resolved issue.
2016 Flagstaff 27VRL Emerald
2014 GMC 2500HD CC 4X4 6.0L