The plywood goes down so the groove is away from the wall, that way, after the row is finished, or at least when you decide to start the next row, a 2X4 can be placed across the joists at the edge of the next row and drive, with a sledgehammer, the second run into the first. The gap should be 1/8" between ends. The tongue and groove (T&G) of the sheets should be driven snug. Do not try to drive them tight, although in some cases it might be needed in order to keep a straight run. The first run is set to a chalk line, but the rows after that can start to meander after a few runs are down, and to correct it with a tighter fit or if need be, a loose fit to correct the run so it remains true
The subfloor adhesive I choose to use is polyurethane based adhesive. It has a long open time, Meaning you don't have to rush to get it covered before the glue "skins" over, thus minimizing the bond. Also, it is twice the cost of regular subfloor adhesive, but you can actually use less. Since there is no "skin" that will form that will need to be broke again by the laying of the plywood over it, to squeeze the glue again out of the skin that was formed by it sitting open before the plywood goes down. Do not drop the plywood on the glue or set the plywood off the final placement too far or firstly the glue will splatter all out off of the joist, or slide or scrape the glue off the joists. I seen both of these things happen in my years of construction. Needless to say the bond will not be worthy. The tongue or the groove does not need to be glued, although there is no reason not to. Some engineers require it in certain applications, but for standard subfloor applications none will be needed. Again it is to your discretion also. If you choose to place the glue at the groove, so the tongue drives it into the groove.
Screws are the best to secure the plywood with, and since a premium glue is used, there only needs to be enough set to draw the plywood to the joists. You can still place them as it has been done before the days of glue which is 6" on the edges and 8" in the field.
With glue you really can spread them out farther, 5 screws used on the ends if you choose, and 4 across the field. There are drills that are sold that use screws that have the screws collated in coils or strips. Makes this go much faster. Rental is an idea here.
Again with the glue, nails can be used too, since the glue will do all of the holding after it has set. Air powered nailers are available for this and will be faster than screws, but the nail gun in some cases does not draw the wood together like screws.
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.
Best Answer
Fasten your first layer plywood subfloor as you normally should.
The second (top) layer of plywood joints should not coincide with the joists and the second layer should not be fastened to the joists.
Article quote from - Position of Underlayment to Prevent Cracked Tile and Grout By Frank Woeste and Peter A. Nielsen For publication by the TILE LETTER Magazine (read whole article):
Also, since you will be using Ditra, make sure to stick with their guidelines for warranty purposes. Specifically see page 8 and 23 from The Ditra Handbook for your application.