-10" is fine. -No, don't try to 'luff' the cellulose. Let the installer add a few inches on top. -I'd install the plywood as close to the top of the cellulose as you can. Leaving a little roomf for air is OK. DO leave about 1" between sheets. This way, any excess moisture and heat can escape. No 2 sheets should be touching.
I'd only do half for starters. And I'd stay away from the gable ends by about 16", give or take. And I'd stay away about 6' from the eave sides too. Heat tends to build up in those areas. By 'confining' your flooring to the 'larger space' in the attic, you get better air and moisture distribution. DON'T stack stuff into the rafter cavities. You want air to flow through there. And try not to pack stuff too close together. By leave 'space' all around the stuff you store up there, you don't inhibit air and moisture flow. And don't 'block' those 1" spaces between sheets either. (They're for ventilation too.)
By only doing half of the attic floor, you also force yourself to get rid of stuff! Otherwise, the attic becomes a sort of 'black hole' over time. My best to ya and hope this helps.
PS: Stuff like plastic and metal toys I store in a large shed. Consider getting one if you can. This opens up the attic w/space.