Underlayment question
Beef up the floor while you can

With 24's and chip board I would consider cutting the floor and adding purlins in between the joists and replace the floor with plywood. Then I'd go with Schluter underlayment.

you wouldn't need to add this many

www.stratco.com

This is a steel shed floor in the picture but the concept is the same, just with wood. 

Then go with double 5/8 offset seams and Schluter. The floor will be thinner and stronger. 

If you want to stick with what you have proposed I would glue and screw everything and hope it's not you taking it up down the road.

Really not a single cut? That's amazing.