What is the best way of saving emails from accidental deletion?

Don't delete them seems simple enough.

If you want to remove them from the server, but keep local copies, create Mailboxes in the "On My Mac" section and drag the messages into those Mailboxes. They'll be removed from the server and stored locally on your Mac. Then, don't delete those, either.

Yes, I completely understood that you don't understand how the IMAP email protocol works.

That's why I explained how to maintain local copies of your emails, while still using the account.

You could also just find an email provider that still provides POP access and switch over to that. That would download all email messages off the server and store them on your Mac. If you need to access the account from multiple devices, then POP isn't really configured to do that well.

Does it work both ways . . . emails deleted from my Mac Mail account are automatically removed from my ISP mail servers?

Yes

In the past, when getting a new Mac I have copied the Mail folder and the com.apple.mail.plist from the old Library to the new one (via an HDD or memory stick. Is this still a viable proposition?

I don't think that is the best way to do it, but I can't say whether it works or not.

I just use Migration Assistant.