1) Jailbreak+Frash works, not sure why it doesn't for you. Another option is using the Skyfire browser from the app store, but that doesnt always work. In any case Flash is dead and any site supporting it is a trash website anyway. Most flash websites detect iphone/ipad and show the mobile un-flashed version.
2) No, you can't install Android or Windows XP because XP would find no drivers and if you want Android, then get another tablet.
Any video you've seen showing XP or Android is sure to be a Skin using Dreamboard or alternatives which is just a UI change.