Noble Company has a new low voc sealant that I have not experimented enough with - this could be a good option if you choose to go with the more premium membrane.
Installing NobleSeal TS around the wall to window jamb is harder than with Kerdi - since Kerdi is so much thinner. Even Kerdi DS is a little tricker to wrap these corners. One trick we do is to set the membrane on the wall first. Let that cure over night then the next day set the side window jambs. This allows us to use bracing and spring boards for slight pressure.
When I say spring boards I do not mean like the jumping platform.
I mean sticks cut about 3/16" larger than the opening and bend into the window well so they push out from both ends. Make sure the installer uses a block to protect the membrane.
You can see here the many sticks bent into position to brace from the ceiling down to the floor.
On a job site we will use anything for this step. Scrap wood - scrap backer board - anything. Just protect the membrane from the spring board's direct pressure. Here on this yacht build I borrowed scrap plywood and hardwood to hold down some low volatge wire into my pre-slope step.