Flooring – Should I level the subfloor before or after adding another layer of subfloor

Lots of questions (related to this Meta discussion). I will just tackle a few points I feel confident about answering.

Subfloor

The Schluter DITRA does not need to be on CBU so I would skip the 1/2" layer of cement board you are planning on using and make your plywood subfloor thicker (or just skip the CBU and keep your 23/32" OSB). CBU is not "stiff" enough to be a subfloor, it can be used instead of DITRA but it must be secured to a subfloor. It rarely makes sense to use 1/2" on the floor unless you are trying to make the finished floor come out to some specific height (for example to help the transition to another room with a different floor). If you are not goin gto use DITRA, you will want some CBU on the floor but the 1/4" stuff is fine.

Apply the modified thinset and DITRA directly to the plywood subfloor. Checkout Schluter's DITRA Installation Instructions for details on the subfloor prep and how to secure to DITRA to the plywood.

Toilet

Without a picture or diagram it is not really clear to me exactly what the issue is with the toilet drain, but that won't stop me from taking a guess :)

You may be right about getting away with cutting a larger hole in the subfloor to handle the bend of the toilet drain pipe. Are you sure the PVC drain will stick above the finished floor? Realize that the thinset between both the subfloor/DITRA (not very much) and DITRA/tile (more so) will add a little to your height, maybe between 1/8" to 1/4".

Have you measured the height of the toilet drain with the flange installed (just dry fit it to take a measurement)? The flange can be installed on the top of the finished floor (tile), on top of just the DITRA, or on top of the plywood subfloor. In your case you would probably want it as high as you can, so install the tile first and then glue the flange to the PVC closet bend. I think once you dry-fit the toilet flange you will see you are OK here.

You should shim under the loose joists with steel shims, not cement, because cement causes wood to deteriorate. Large galvanized washers and small steel plates can be used as shims. The framing block in between the joists are called bridging and in an old house I would space them about 4' on center. You can off set each row by 1 1/2" which will enable you to blind nail through the joists which minimizes toe nailing. Use 3/4 tongue. And groove ply glued and screwed to the joists and bridging. I would use pl400 for glue and 2" exterior screws spaced about 8" apart. If a second layer of ply is desired you should also glue and screw it the same as the first sheet. Try to hit the joists. Use a chalk line to mark the joists. If a solid wood finished floor is being installed 1 layer of 3/4 plywood is all you need.if a ceramic tile,stone or wood laminate floor is going in use 1/2 ac plywood over the sub-floor.