1) No one knows, we haven't gotten an answer yet.
2) Its probably crappy sealing jutsu, so not much use to Orochimaru.
3) Konoha likes having allies and is desperate in that way..
4) Filler episodes are bullshit, don't base anything on them. Hand seals allow you to mix chakra in your body(explained in like episode 5 or something by Sakura)
5) Familys have unique techniques that require something else in addition to hand seals (like certain chakra mixing techs or something) They are not bloodline limits, just secret techs.
6) Filler episodes started around like episode 130 something, they'll end eventually, and you'll get your quality anime back. Btw, it seems Bleach is heading to fillers now, so we're boned. Bleach 63 special is on dattebayo's torrent site.