I have a roll of Schulter ditra (with the dimples) for future use as underlayment in bathroom. Can this be used instead of the traditional felt and lattice prior to doing a mud pre-slope. Everything will then be covered with the Kerdi membrane

I guess it could, but I don't know how you'll get the mud worked into the cutback cavities properly to make it worthwhile. You would need to use thinset to properly fill in the cavities and bond the mud to the Ditra. Of course, you'd also need thinset under the Ditra.

Around here, metal lath is only 50-60 cents a square foot, about 1/2 to 1/3 the cost of Ditra. Of course, you don't have to use roofing felt underneath, you can use plastic of any type. Its purpose is to keep the plywood from the drawing the moisture of the mud. You can use some cheap painters plastic or even a garbage bag.

I had already purchased a roll of Ditra for use on the surrounding floor outside the shower and will have a surplus of that material, thus my thought for using it vs. felt and lath. This is my first DIY shower job so I cannot speak from experience but it seemed that Ditra offered some advantages vs. felt and lath. Getting the mud to embed into the dimples is a potential problem?

I don't think it would engage the dovetails like it's supposed to. Thinset being a much thinner mix will, but deck mud is much more sandy and dry.

If I had the choice to use Ditra or nothing in this application, then I'd use nothing. Maybe just some plastic on the floor. But I'm not condoning using nothing, I just think Ditra will not serve any more use than a piece of plastic.