I want to open Mail as Read Only, but it forces me to choose/create an account

The Mail App frequently upgrades mailbox storage between macOS versions. Since the mailbox wasn't online it couldn't authenticate and therefore couldn't upgrade the offline mailbox. Ideally, it would have been advisable to move the old mail to be in a folder under On My Mac which is where one would archive old mail.

But now you are in a scenario that would be difficult to revert. Here's a suggestion. Setup an email account in Mail, doesn't have to be Gmail, could be any other email account. Then see if you can Import Apple Mail and point to ~/Library/Mail by pressing CMD+G and typing in "~/Library/Mail" without the quotes and pressing Return. Then choose the Mail folder. It should scan and detect the mailboxes and allow you to import them.

Well, "On my Mac" is new information you did not provide. The workaround is to add any email account and then take that account offline and you will be perfectly fine accessing On My Mac email archives.

It's fine if you choose to use another email application, that is your choice. You might wish to get Mail working first then export your email.

I understand that Apple removed the option to cancel the prompt to add an account at first launch. But complaining about it here is unhelpful. This is a community of Apple customers. You need to send feedback to Apple Developers:

https://feedbackassistant.apple.com

There is a strange workaround. Add a fake e-mail address, let it fail to connect, and then it will let you cancel.

It did work for me last week or so, it was more than a "maybe".

I wanted to test Mail on a second user account (something related to a question here at Apple discussions). I failed to write down the exact details, but I do have Mail working without a functioning account on my test user.

Many more people use cloud based solutions and we really do not keep email local nor offline anymore. But I get your point. Sounded like you wanted to access an old email account that would no longer connect versus archived email in On My Mac.