hi i have been practicing the art of parkour for about 3 years now and i have increased my level allot and think that i can give you some helpefull advice for your parkour.
1. i know someone that has flat feet and he can do high falls and flips and what not so i dont think that it will affect you to much but i might ask your docktor or something and see if that kind of physical activity is ok.
2. as long as the arch support is soft and not hard it will be ok, just watever feels most comfortable i actualy use arch support to sometimes so i dont think that it will be a prob.
3. i live in oklahoma and my crew and i go downtown and run every weekend and when i am not in okc training i am at my house training on home maid vaulting rales, pressure land gaps, cat leaps, tick tacks, flips, balance, and mental as well as physical strength, so all in all my physique and training is pretty important to me, and if i were you i would start with baisics, no life or death jumps, and gradualy increas at a pace that is most comfortable for you

if you have any questions on how to make home maid equipment that just ask ohh and i also go to a gymnastics gym and train there too for the moor complicated flips and trick and what not
4.doesent realy matter as long as you do tricks that are not to advanced, like if you can jump 8 ft. dont try a 9 ft. gap pretty much common sence
5. wheights dont matter as much as physical(own body weight) training does but they can be usefull i would recommend just going out and seeing what your stenghths and weaknesses are then bais your workouts on that
6. i advise athletical tape and you can buy special ankle braces or wrist, elbow etc. so you can try some of those that should protect you enough.
7.whenever you feel most confident in your skills but even if you have not conditioned at all and you are slightly fit for your body size than the baisics vaults, gaps etc. should be able to be drilled and practiced.
8. i realy like davidbelle.com, cyrilraffaelli.com, chasearmitage.com, urbanfreeflow.com, and r-evolution.be.cx