Sitewide Comment Control

No. The sitewide lists do not edit the per-site lists.

They remain blocked for that site, but only that site.

This can be a little bit of a shell game. The answer is 'both' and 'neither'. The act of ''blocking'' a user takes precedence. This means if a site on the network marks a specific user to be blocked, even if the network option is set to spam, the comments will be blocked on that site. On the other hand, if the network blocks someone, that cannot be overruled by any site on the network.

Not at this time.

Is so far as WordPress itself keeps rejected comments, yes. If you set comments to be spammed or moderated, they will show up in the appropriate section of your comments page. If you set comments to be trashed they get added and go right to trash.

Usernames, IP addresses, and email addresses.

Yes. If you put example.com in your list, it will block anything@example.com. However it will also block someoneelse@thisexample.com so use the @ when specificity is needed. Be very careful when you do this! If you put in just the letter a for example, you will block all email addresses with that letter. I strongly recommend you only use full domains ('example.com', not just 'example').

Yes, but it's optional because it's an experimental feature. The logic is a little chancy and has a higher risk of catching innocents. However if you add spammer@example.com and turn on Wildcard checking, then spammer+avoid@example.com will be caught, but spammer+another@gmail.com will not be caught. The downside to this is that spammerama@example.com will also be caught. Use at your own risk.

This plugin will not block any logged in user, even if their email is on the list. If you don't want them commenting, delete their accounts. It's assumed if you let someone on your site, you mean for them to have access.

That's a tough one, I hear you. I recommend you reset their password and change their email to something invalid. Then you can add their old email to your block lists as needed. Sadly there is no plugin I know of that handles this for Multisite.

Yes, it does.

No. But then again, you don't need this on single installs.

Github - pull requests are welcome.