For your second question, it is possible to add multiple email accounts in Thunderbird, including Gmail accounts. After adding your first email account, you can go to New Account -> Email to add any additional email accounts that are needed.

And the first question I think i have answered. The disk image cannot be opened - it reads <corrupt image>

Does anybody know of a Thunderbird version that will work on OS Mojave?

Mojave is supported by Thunderbird ESR 115, but nothing newer https://www.thunderbird.net/en-US/thunderbird/115.0/system-requirements/

You would want to install https://www.thunderbird.net/en-US/thunderbird/115.14.0esr/releasenotes/ from https://ftp.mozilla.org/pub/thunderbird/releases/115.14.0/mac/en-US/