What size is the tile you are installing? I love 254 and it is one of our favourites - Schluter wants you to use a non-modifed thin-set over Ditra to play by the book. Over Ditra when we want a modified thin-set we have been specify S14 or X32 from Ardex - these are self curing thin sets. Setting Ditra to plywood 255 is great since it is a top quality modified thin-set.
I set a brick to Ditra with 254 Platinum and the Ditra to the plywood mock up with the same. I hung over 1600 pounds of water from this test. The larger the tile the longer it will take to fully cure underneath. So pick your thin-set based on the time line you are working with - or swap out Ditra for Strata Mat.
Have you seen Laticrete's Strata-Mat? It is perforated and allows for modified thin-sets to be used over it. Strata Mat is an uncoupling membrane.
As for the niche how have you designed the tile layout. We like the tile to wrap into the niche so most times we size the niche a little larger and adjust the size once we tile to this point. We have worked from the niche out before so anyway you tackle this you should be fine.