In all previous versions, you could also open the Address Panel from the Menu (Window/Address Panel) - but that has gone in this release.
Even using that blue plus sign, there seems to be no way of multi-selecting in one go. It's one at a time.
I often have to send emails to 50 or so people, most of whom have multiple email addresses (Work, Personal, etc). And often it's just a one-off, for a meeting etc, so not worth going into Contacts and setting up a specific Group. With the old Address Panel, there was a column of Groups, and the other column was just a complet but simple list of all contacts (or contacts in the group if a Group was selected), along with all their email addresses - nothing else. And three buttons up the top for To:, Cc: and Bcc:
I could skim donw the list, multi-click (Cmd-click) on each person I wanted to send to, chossing which email address for each one that I wanted to use, then click one of those buttons at the top. Easy-peasy.
Now there seems to be no way of doing that. The blue plus sign thingy doesn't allow multi-selecting. As you said - it's one at a time. A complete PITA.
Can't even do it in the separate Contacts app (which I don't want to have open all the time anyway, due to screen real estate on my Macbook Air (I'm typing this on an iMac that I haven't upgraded, and won't be). Contacts app doesn't show you a list of contacts and email addresses anywhere - it's one at a time, with the full card of info for each contact.
Mail has become the only email app I'm aware of that can't access - properly - a list of contacts and their email addresses from within the app itself.
Submit the issue to apple.com/feedback ..... I have and several others have. The more that do it, the better. Even if Apple does have cloth ears..!