1. Is 27.0 going to work with this version of BA? (somehow I didn't see the later versions when I started). I.e., should I start over?
You have to turn off pruning in these versions of Core (set "prune=0" in bitcoin.conf), or it will discard the earlier chain history after parsing it. Armory needs to read the whole the chain. If you have less than ~100GB of data in your current bitcoin data folder, you have to start from scratch.
2. Can BA scan the core so far? This is an old wallet so I think there was activity by 2018 which is what it is up to. How do I see balance/transaction history up to this point?
Armory reads blockchain data on disk so if you have data up to 2018, it would should your balance at that time. If something happened that balance in say 2020, Armory would not be able to see that until you download the blockchain up to that point.
3. Will BA automatically "talk to" the Core during or after download? If not how do I connect them?
Armory reads blockchain data from your disk, it does not need to talk to Core to do that. Matter of fact if you want to sync to partial history, you're better off shutting down Core once it has downloaded enough data. This is so that it doesn't keep on writing to disk while Armory is reading the data. Armory needs to talk to Core directly to broadcast transactions however. It does find Core on its own without your intervention, but you'd get a better experience if you set "server=1" in bitcoin.conf.
4. I successfully restored an old wallet from a friend's paper backup, can I use some piece of the wallet info on another site to see if there are any transactions or a balance?
You can look up the balance on each individual address in the wallet by pasting them into the blockchain explorer of your choosing.