Lots of questions
. I'll offer my two cents worth, but don't be surprised if you get many differing opinions on finishing. Ask 10 woodworkers a finishing question and you will get twenty or thirty good answers (and maybe a few crappy ones
First of all, if there are no defects (scratches, marks, etc) in the plywood, there is no reason to sand very much at all. I usually stop at 180, or 220 if I am going for a really glossy look. 320 and above are best used after a finish is applied to scuff up the coat to improve adhesion for the next coat. It is very easy to sand through that top coat of veneer and very hard to fix.
Your combination of woods is unusual. What do you want to accomplish with your finish? Do you wish to accent the different woods or to help them blend together? The most malleable wood is the maple. A clear water based poly will keep it from darkening, but may make it look plastic-like. If you want to warm up the maple and darken it a bit, an orange shellac might help. If you really want to "pop" the grain, start with an oil based product, maybe oil based poly.
Lots of choices. Let us know more about the look you are going for and maybe we can offer more useful advice...