[Can I see all the accounts registered to a single email address (mine)?]

if you go to https://secondlife.com/my/account/request.php?lang=en at the bottom there is a "forgot your name?"form which you enter your email address, i am not sure if that does email all the names associated with that email address though, worth a try!

[Stuck with a "TG" profile!]

Your profile is indeed still that of a teen, ( not even showing up at all for adult residents). You need to open a support ticket to get that corrected.

[Giving inventory item]

I am not sure what the billing team would have to do with this.. but anyway, you can open a new inventory window ( file > open new window in inventory tab ) then drag it into the other person profile on the side bar