zetti you are becoming "over-informed" and confusing yourself maybe.
First of all as already stated...all of the Schluter products are available at all of the Home Depot stores. Some of it is special order but they have it all.
Durock can be installed with either modified or unmodified thinset, unmodified is less expensive and preferred under cement board by the Tile Council of America.
Premixed mastic should never be used on a floor, doesn't matter what the maker says, it's never a good idea. Mastic is a higher profit-center for retailers by the way so they like to sell it.
Unmodified thinset can be used to install the tile over Durock and is mandated over DITRA.
If the tub surround is also a shower then use the modified thinset mortar they sold you.
Return the mastic, it is basically usless for your applications.
Porcelain tiles ARE ceramic tiles.