Wood – Do I need 3/4 inch plywood subfloor

If the subfloor is plywood no other ply is necessary; but you do need a flat surface for the Ditra. If the subfloor cannot either be scraped, chiseled, or sanded flat then self leveling compound may be required. If you use self leveling compound remember that the surface should be primed first - follow the product's prep recommendations. 1/8" plywood, in my opinion is not sufficient; unless it was set down on flat surface with troweled adhesive and stapled 4" o.c. I would recommend 1/4" min - which can be found at most flooring supply stores - specially formulated for flooring; i.e., all voids filled in the plys and water resistant glue.

The 45 degree decking is the primary sub-floor. It's a technique that was widely used in the past, but has subsequently been replaced with simply dropping sheets of plywood down.

In answer to your questions:

1) You can glue and screw to the sub-floor.

2) In this case, it doesn't matter. The sub-floor is carrying the load diagonally. However, if there were no sub-floor, then you would want to go perpendicular. See this discussion on the subject.

3) Always - always - always level as you go. Fix the level at the soonest possible opportunity or it can be amplified by an order of magnitude.