Hi, and first off, welcome to PA!
I'll take a shot at answering your questions.
1. Assuming its not too extreme, yes you could use it. The concave shape you mention is what we call either reflex or deflex depending on which direction it bends. In the case of a board, you can basically choose which you want it to be, which brings me to the next point.
1a. You would attach the riser block to the convex side so that the ends of the board are bending away from the handle. This is reflex. If you did the opposite and the ends of the board bend toward the handle, this is called deflex.
2. I interpret this question as does it matter if you establish your width taper before or after your initial thickness. If that is correct, then the answer is no... sort of. What matters is that you get the bow bending evenly througout. Everyone develops a different process, but the majority, I believe, lay out their width first leaving themselves a bit of a saftey margin, and then establish an initial thickness taper prior to tillering. This is the way I do it.
I'd like to add that you should glue on your riser block before reducing your thickness and shape it all as one unit. Otherwise, if you glue a riser block onto a 3/8" thick board, you're nearly certain to have it pop off when the bow starts bending.
3. If your red oak board truly does have good straight grain, then I don't think it would need a backing. I've made several RO board bows with no backing. But, being a beginner, a rawhide or linen backing would be good insurance against both mis-reading the grain and tillering errors in terms of personal safety. Neither will prevent a bow from failing, only from harming you if it does.
3a. Although I've never tried it, many have reported using rawhide dog chew toys to back a bow. I prefer linen cloth. Both are relatively inexpensive, but real linen cloth may prove to be difficult to source locally. I ordered mine online.
I'm sure you'll get several more opinions and I hope mine was helpful.
