Use modified to put down Ditra over plywood. Unmodified on the top. Joists 16" o.c. with 3/4" plywood type 1 exterior grade. You can wrap inside corners as long as you maintain the 2" overlap. Kerdi membrane is thicker than Kerdi band so it may increase your build up. Sorry I can't help you with box store product advise. I'll suggest buying from a professional floor covering store. If you are using a large format tile medium bed mortar is required such as Mapei Kerabond T.

Thank you, I think I will use the ditra set, its about three times the price but I was warned about box store products and after spending all this money what is 10.00 more. We have a local floor and décor and they sell it I think. Ok off for more questions.

"i am using durrock, and on the exterior wall I will put pink insulation and pull off the paper face."

You can actually leave the paper face, as it's a fairly poor vapor retarder and not a true barrier. If you still want to increase it's ability to pass vapor, then just slash the paper facing a few times with a utility knife.Or you can do what you propose.

"I skimmed the durrock first with unmodified thin set to make installing
Kerdi easier due to dry out problems. (Right unmodified?)"

Moisture wicking ismore a problem with fiber-cellulose products like Hardie. At least in my experience. But if you skimmed the Durock, no worries.

"...can I bring 2" of it to adjoining wall then when I do that wall bring 2" to first wall."

If I'm understanding you correctly, the Kerdi is about 39-1/2" wide and you have a roughly 36" shower, so you want to run the membrane through each corner by 2"? Instead of doing that, I'd start the sheet just shy of the first corner, let it run across the 36" wall, run through the corner, then run 3-1/2" or so on to the next wall. When you hang your second sheet on the second wall, again start it in the corner and do the same. This will give you a roughly 3-1/2" overlap on the flat of each wall instead of having two thicknesses of membrane overlapping in each corner.

Yes, you'll want to use unmodified thinset to set the large format tile. Home Depot in my area sell an ummodified called MasterBlend thinset by Custom Building Product. I wouldn't use i unless you also bought a bag or plain portland cement and added a shovelful of portland to each bag of MasterBlend.

Lowes in my area sells unmodified Kerabond by Mapei. It's pretty good.

Be sure to use the Kerabond with the "T" for large format if you go with Mapei.

Thank you both so much....mondo your tip about pulling the excess to other wall is giving me a why didn't I think of that....thanks for always being the Pro, both professionally and personally.