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.
I have seen it fail. I have momentarily tried it. And I will never waste my time again.
Fail
Went through a potential house 8-9 years ago. Mid 60s house. They just put down hardwood to sell it. A floor that had just been laid a few weeks earlier had shifting in at least two areas. I popped off a piece of trim in a closet. Pointed flashlight down, stuck screwdriver in and broke off a piece of particle board. I ended up talking them down 15K on the house because of the issue which I reminded them they would now have to disclose.
Two workers and myself had all of the wood popped up (1000 sq feet worth) in about 3-4 hours. I was using a putty knife in some places. The staples didn't hold - and they were long. We took the particle board out and threw down plywood... Probably the most tedious task was pulling the staples and numbering the planks. We got 15K for 2 days of work.
I tried it....
A couple years previous to this I pulled up carpet out of a house (lush wool from the 70s that was past its time). I had a few hundred feet of leftovers from a previous house so what the hell... In my third row I was push my first row over... I stopped and retreated.
Particle board was used a lot in my area in the 60-70s for carpet. I can see it working for anything else. Hell I am not even sure how it lasts 40 years in some houses. When you pull it up you can see it swollen in some areas. Even when screw down it pops right up. Nothing sticks in it (don't get me started on glued down particle board which is a disaster to demo).
Take a couple planks. Staple them down and watch how fast they move.
I think you have some options... Go back to carpet. Go to a floating floor and make sure you have a really good underlayment (semi-waterproof). Or take out particle board and replace with plywood. If you are by yourself on the job it could take a while but with a couple of helpers it could be a one day (long day) project. OSB is a subfloor that I would really be wishy-washy on installing hardwoods (never in my house) but particle board is never a thought to me.
Best Answer
You should be able to gently tap the area close to where the hole is and listen for the hollow sound that a thin layer of plywood over a void would make. Then using a 1/8 drill bit probe the area to locate the center and cut out a section large enough to give you a look inside. After that gently cut away the rest to the edges of the vent hole.