Cut a board to the wall and prop it straight mount your flange and backfill the water plug hydraulic cement. Within 5 minutes it will turn solid give it a days rest adjust your flange slots off the wall to center and tapcon the puppy to the floor.
I thought of another idea.
Everyone says to back fill the hole with gravel and top off with concrete, minus the gap around the pipe for the flange. If the pipe was perfectly level, that's what I'd do.
Since it's not level, rather than back filling with gravel, I could do your method and fill with Fast Plug below where the gravel would be. I would fill it up enough to where the Fast Plug would be about half way up where the existing concrete slab is. That would give me enough hold to keep the pipe in place.
The remainder of the fill would be with concrete up to the level of the floor minus the gap around the pipe for the flange.
The only downside to this would be the elbow and the majority of the pipe will be encased in Fast Plug rather than gravel. If I ever had to tear it out.....good luck.
Is this what you were suggesting in your top post? Fill the bottom around the elbow, etc with Fast Plug?