Tom,
Correct, Ditra's minimum is 5/8" plywood or OSB. If you have access to any of your flooring from underneath, look at the underside and you should see manufacturer's stamps indicating plywood size and type (e.g, PS I-95 16/32", exterior glue). Of course, this only tells you the bottom layer of ply if there is more than one layer. If you can't tell the total plywood thickness by looking/measuring where any pipes might pass through the flooring, you could also take a "core" sample by cutting out a hole core from your flooring in an inconspicous area (using a 1/2" hole saw bit). If you have the height available for more plywood, I would add at least 3/8" exterior grade, BC plywood and then your Ditra.
As for your PVC toilet flange, while you could use flange extenders or extra wax rings, a better solution would be to cut the pipe and then reinstall a new flange tightly screwed on top of your finished floor.
Good luck,
Greg
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