Hey, welcome to the site, it's rad that you're looking forward to making a char and hopefully myself and other members of staff can get you the answers you're looking for.
1.) It's 15 posts per topic (or 12 if your Bijuu doesn't have Godlike Stamina). Pseudo-Jinchuu have a refreshing pool of 80 additional Stamina per topic and follow the same rules as the V1 and V2 transformations listed here
. I don't believe you need a seal though if you wanted one for story reasons I'm sure that wouldn't be frowned upon.
2.) I don't believe so, not without a Restricted to give access to Sage Mode. As for drawbacks it would depend on the benefit so it's not the easiest thing to answer.
3.) I also don't believe so (as someone with Gelel Stone I have asked this myself and the general consensus seems to be no).
4.) That simply probably not, however there are instances where you could learn/steal a technique/item from another PC or something along those lines. Taking someone's eye if they have Rinnegan is another example, things like that.
5.) It is not approved for use and I'm not sure about helping to facilitate an approval as it was problematic in the past from what I understand (was before my time here).
I hope these at least somewhat helped and if you have further questions or follow ups on anything I said or missed, please feel free to ask.