I too have found a lot of the deleted ones to be at least a month old. I have not tried going to the cloud - felt it would confuse me further! Nor have I tried to transfer an email with attachment to another, though I can forward one with attachment.
Since I asked this question, some friends said that emails don’t disappear when you "delete" them - they just tell the algorithm that you’ve read it (it does open the first page of that email when you select "delete"), which keeps you on its list. You have to get it into your Spam folder. Seems this would take a lot of time?
I have not tried this yet.
Also, supposing you might want to hear once from something (say, from a charity you support) - but not everyday in perpetuity!!!
I have taken myself off many email lists (they give you a link (very small) at the end of page. I get an email back telling me I’ve been removed - but I never am.
Sorry not to be more helpful - maybe an Apple expert will turn up who is willing reveal this well-kept secret!