Welcome, Bobby.
1, 2. Not sure what you mean by gaps here. You're talking about gaps between sheets of subflooring, perhaps? Or are you talking about dips in the subfloor?
3. That's gonna make a pretty substantial hump in your Ditra installation unless you do something about it.
You should go to the Schluter Ditra website and download their Installation Handbook and read all you can about your installation. They go to some length to tell you precisely what types of thinset mortar they want you to use to install the Ditra on your wood floor and what to use to set your tiles. Up to you then what you elect to do, but for absolutely sure you must use a modified mortar meeting the requirements of ANSI A118.11 to install the Ditra.
My opinion; worth price charged.