Thanks a million to both of you. These things have solved most of the problem.
This one point was 99% of the problem.
I checked some old screencaps and I had previously had https for my main blog... then it wasn't there anymore. I don't know how this happened.
A few months ago I mailed DH support and asked why I could no longer get https access, and the reply was "Your DNS is broken, read the FAQ, also I noticed you don't have an htaccess file, so made one for you." When I replied that the DNS FAQ didn't change anything and adding htaccess file didn't work, I got another email saying I should check my DNS settings and by the way I didn't have a htaccess file and that this different support worker had added one for me. At that point I gave up dealing with it and worked around it. Today I noticed the htaccess file that was handily created for me (twice) was empty.
So the aliens appear to have deleted my htaccess file at least twice, replaced it with an empty one, and possibly, somehow deselected 'Secure Hosting'. Or maybe there was a wrinkle in my hosting somewhere there.
Either way, that's now fixed. The .htaccess tweaks from that page has the redirection going swimmingly. Thanks again, friends.
ownCloud app still doesn't work, but I'll continue to work out what it's problem is. I think it's the app.
New Question: Lastly, I have six WordPress blogs going on the one domain and half of them are verified by Let's Encrypt and the other half aren't. Anyone know how that's possible?
Thanks again for your help!