1.) Yes those are the only two sizes that are offered when choosing handlebars. Remember, if you buy 1 1/8" bars, you are going to need to order bar adapter's as well.
2.) I run mattoon machine bar clamps but there are a lot of other choices on the market. JDS customs may be a good place to go look for some.
3.) Peeps that run a crossbar are probably trail riding or mx'ing. They run the cross bar just in case they eat shit, they have a little bit insurance. I personally throw them in the trash because I don't like the look of the bulky thing AND I mostly drag race.
4.) I closed my eyes and selected a bend when I ordered my pro-taper bars. I was like you, I didn't have a clue as to what bend I wanted. I chose the CR-High bend. When I mounted them they were too high for my liking so I rolled them down a little bit.
5.) I really like the ODI grips. You don't have to fuck with grip glue, tape or hairspray. You get quite a few different choices of different patterns.They slide right over the bars, lock down the allen head screw and go.
6.) I don't run hand guards, never have. If you XC, MX or trail ride I think they would benefit you though.