I'm using the plywood method anyway because I just bought all the wood last night and I'm not carrying it any more. Plus, I cut my first sheets!

I'll update on Monday :)

Cool, it's a little messy but you get to play in the mud again like when you were a kid! it's easy and will turn out fine.

Not if you key it into the existing floor. Should be done with staggered seams anyway, as with any sub floor. Another way to do that is to start the new sheeting back far enough to be 3/4" (if that's the subfloor material of choice) below level and fill the rest.
5/8ths or even 1/2" will work. You assume the sleepers to be on 16" centers. They can be on 8' centers or sold all the way across. Of course that's not practical in any sense.

My experience tells me each sleeper has to be cut one by one. It's not like gang cutting rafters. A power plane is a handy tool is this situation as well. Just gotta make sure the fasteners are below the projected cut. Some will use adhesive and pull the screws the next day to fine tune the floor. I've seen guys run adhesive for 24" and then a spot of CA glue to hold the lumber while planing before the screws are run. Or 5 minute epoxy.

Remodeling is creative. New construction is boring.

^ I follow you. I've leveled surgically too but you and I do this for a living. We're talking to a do it yourselfer and I belive this will be simpler way for him