I’m not sure how to read the diagram, but if the 2-15/16” piece is wood, couldn’t you just put a deeper piece in and support what you need to support with a skirt against the drywall? Sides of sill(?) could wrap around side drywall and windows trimmed out to your liking.

Talk to Marvin about the Zip-R install. Our window manufacturer would not allow the nailing flange to go into the Zip-R board with the foam behind it (between the Zip green board and the stud behind it). Thus, the framing of the windows became more "involved" and we ended up going with separate foam board insulation to make the install easier. I can go into further detail if you need/want.

As far as the sill depth. Do they have extension jams available? Usually if not, the builder can make them look pretty seamless.

Ohh no - have not heard about any issues with installing on ZIPR. We would build an extension on site we were just looking for something like this.

looking for photos of someone using Elevate....

Either make your walls deeper or have stools tgat stick into the room further.

If I am reading the detail correctly, the 2 15/16" piece is your jamb extension to a 6 9/16" or whatever it is. Replace that piece only with the size needed and use an apron underneath to support it.

This is most of what kelli_ga said above with some clarification.