I am sorry for the long delay but i finally replied to the mail.
Hopefully most questions are cleared.
As question 1) might be interesting for other users as well, i will post my answer to that question here, too.
Quote from Xmen
1) The API required to verify users is insecure. It can easily be used to brute force email addresses. It is likely that we will need to work out some other more secure API.
The current API should expose no security threat.
You can not link other people accounts by guessing their email address. The linking has to be confirmed via email.
The only thing you could do is guess username/email pairs to reveal someones identity.
The validation URL is kept secret, and you can use any name for script and get vars on HTS.