Q1) Is gitlab a better source host than github (or google)?

Yes, because it isn't owned by a malignant corporation and because it's open source.

Q2) How do I silence the mail queue?

You don't want to silence it, you just want to remove the messages which are stuck in the queue and will never be delivered. qtool.pl is probably best for this.

Q3) Should I substitute sammy for root in the above?

You could if someone is going to be sure to read sammy's email. So long as someone is getting and reading the postmaster emails it can be any user.

Q4) What does /etc/aliases look like on a working install?

I'm not sure what you mean by this. The format is as described in the man page. Are you asking for someone to post an in-production aliases file?