iMac Mail: Accounts offline; re-added account not seen

I believe my mail may be badly broken. I am running 15.7.8 and cannot upgrade to Tahoe because of software incompatibilities with my CAD software. For the last month my imac has been installing 15.7.8 update weekly, sometimes it crashes and I have to run it in safe mode to get the imac working.

In the last month all my email accounts, except icloud, have refused to work properly. They all start offline and after inputting passwords again they connect briefly, but a restart losses connections to the accounts again. I deleted one email account and added it again, only to find that whilst listed in the system settings, the mail app did not recognise it. Instead, every time I opened mail it showed an import folder and imported all the emails from that account. Numerous imported folders being listed was slightly inconvenient!

I have searched for all the mail preferences, plists, containers etc looking to reset them and they are not in the standard locations. In fact, they don't exist in any location that can be searched visually!

Other than a complete reinstall, which is a pain as I have to force it to an earlier version, is there anyway to resolve this issue?