Answers on what I know, not much about henna:
1. RC is considered a semi/demi/permament color. The permanence depends on the length of time it's on the hair.
2. Yes, it covers gray! I have used it for over 13 years to cover and blend gray in my hair. My older sis has much more gray and you'd never tell it by her excellent haircolor!
3. RC Color is much easier to use than henna: no scent, not messy, only takes 15-45 min. I haven't used henna myself but I believe it's left on overnight in most cases. RC will not dry out your hair. I have heard henna can dry it.
4. Henna will color your whole hair, as will RC. But if you choose an RC shade lighter than your hair, it will blend the grays without darkening or changing the tone of your hair.
These next ones are better left for the henna gurus:
5. I think you can add indigo to counteract some red, but I believe it's still going to be reddish.
6. and 7. - no idea!
You CAN just cover your gray with RC Color without changing your haircolor.
If you decide to go this route, LMK and I can help you choose a perfect shade. I've had great experience helping curlies find the right shade over the years.
FYI: However, I believe that you CANNOT use RC over henna, so if you go the henna route, you'll have to wait until it grows out to try RC. I think you can do the other way around though, RC then henna later.