Deleting a mail account on only ONE iCloud synced Mac
The answer turns out to be the no, you can't delete an account on one iCloud synced Mac, without deleting it on the other(s). Beyond leaving said email account disabled, you must go into ~/Library/Mail/V5 in the Finder and uncover the multi-digit-letter folder associated with that account and trash the contents. That's how to reclaim the space occupied (in my case 15GB).