I finally got time to weld up the the flange crack on the Glenbrook's polypropylene tank tonight. It was a pain getting everything out of the way (plumbing, propane line, crossmember) but it went very well! I'd say it's better than new at this point, as it now has a proper fillet for reduction of stress concentrations, as well as mesh reinforcement. It was very tedious, and I think I probably used up half a dozen PP filler rods. The Polyvance airless welder ended up being the only tool one I used.
The crack ran all the way across the top and around each corner a bit, mostly right where the flange was welded to the tank shell. Honestly, the factory weld didn't look all that great. They definitely do it quickly with some sort of extruder laying down beads, but there's no blending to the shell. No wonder it cracks with such a thickness mismatch. It did last quite a while, and the PO did indeed mount things without supporting them well, so I guess I can't complain too much.

I grooved out the crack with the hot iron, and filled it with the melted rod. Then I filled the valley with more material, smoothed it over, tacked some mesh reinforcement in place, then started working it into the fresh surface from the middle of the bead outward, wrapping the flange edges and spreading up onto the original tank surface. This flange feels SUPER solid now, and no longer deflects like it did. I think I'll get one of those inexpensive flange adapters like
@tmsnyder used, and use a Fernco to go from there. No more stresses on this tank! It should last indefinitely from this point forward.
Once it was all structurally acceptable, I kept the surface warm and did another skim coat over everthing to ensure the mesh was well buried everywhere, but it looks like I didn't get another photo afterwards.
Oh, and I dug a large McDonald's cup out of my friends trash and cut it down to plug the opening (funny enough, it was PP so I could've even tacked it in place if I'd wanted). That really helped cut down on the smells since it was a very close-quarters repair.