Tore up the wall to see whether I can adjust the carrier (the waste horn is 1/2" too high). Of course this kind of, ahem, 'carrier' cannot be adjusted. I posted a photo. I see two options:
#1 Rip out all the wood, cut new wood and screw it in again. Kind of a pain because I have to tear stuff out of the framing. Plus I don't trust it much with a heavy toilet like a Glenwall.
#2 Re-plumb for floor discharge. This has three challenges. The floor has a thick mudbed and it'll destroy the tile. Oh well, I don't mind tiling. The 2nd is venting since it's tight down there and I'd have to 'detour' the old stack around the floor discharge pipe to vent downstream as code requires. Can I run 2" vent down there and tap it in or does it have to be 3" up to the toilet trap height?
The biggest challenge with #2 is that I'd like to get rid of that little addition. The ABS is really tight in there and I don't know how to saw it off and cap it so it still acts as a vent, is per code and won't protrude into the new drywall or Hardibacker.