Seriously? You are taking up a permanent floor to put in a floor that will last a year? Whatever for? FYI, the toilet is the first thing to be removed when working on a bathroom.
Start against the longest outside wall, leaving an even 1/2" space. No leveling necessary with those shorts. Sanding? No more than 100 grit screen on a buffer. The toughest part will be getting the dust out of the bevels afterwards. You do realize that you can't buy good stain or finish from a...
That is called the picture frame effect. After sanding you need to use a 60 grit screen on the buffer and then a 100 grit. The entire floor needs to be sanded with the same grit. Changing to a lighter grit only makes it worse.
I was confused since you used "underlay material and staples. If I buy Aquabar" in the same post. And I have never heard of underlay material. If there is no basement, of course I use it. Moisture is the enemy of wood. What do you sell their? Prefinished 3/4?
Rarely is a subfloor flat. Next time you see one, hold your 6 foot straightedge over one of the seams. You will see for yourself. I always check the seams before installation.
Paper & staples usually , but never glue. I only supply what I can buy wholesale or in bulk, and these glue manufacturers are lazy. I cried when they discontinued Franklin 711. That was some great adhesive.
Oh crap! two footers on the diagonal? Ask for $1.50 extra. The biggest I ever did on a diagonal was 18" I had to cut halfway thru the turn them for the other half. Took a lot longer.
I have a ligno. 6% is fine unless the job is on the ocean. That is a normal reading for a dry subfloor also. The Aquabar will do fine. I use 40 60 100 and screen with 60 again for a drum sander, skip the 60 grit screen if using a belt sander, then screen with 100. I get by with 60 -100 on the...