6mm prusik (the UIAA rate stuff) is plenty strong to support body weight. Have used 5 and 6mm plenty of times to prussik up and to make haul systems. I even learned to use 6mm cord to escape a sport climb route. Having only the main rope connected to the bolt by a 6mm prussik and rappeling of. (NEVER lower off a prussik or sling, failure is almost guaranteed) I do prefer to leave gear, than doing that.

I never learned to double or triple wrap a figure of eight. So can't comment on that. Haven't tried to use the eight upside down either. Just seamed really tricky to me.

As for carabiners strenght. It's should be printed/stamped/forged/lasered on them. I only carry 2 big HMS biners, the rest are all small lockers. As long as you can fit a proper clove hitch in it, it will be big enough to do most jobs.