1.) No idea what pound sign does yet

2.) There are 4 privacy settings on the lower left hand side of the box you type into. Public, Unlisted, Followers Only, Direct.

Public: Everyone can see
Unlisted, as in not viewable from the time line
Followers: Only your followers can see it
Direct: Only you and the person you pinged can see it, pinged as in @'d them

3.) The feed is in the global or local timeline as in, global will allow you to view everything (unless the server your in blocks an instance for being abusive).

4.) Instances are other servers hosting the mastodon software with a domain name as far as i can tell.

5.) no idea about your kinks? o.O