5. 520 errors were through CloudFlare. If I can create them again and see them without CloudFlare I'll report back.
Excellent. 520 errors don't tell us much, so as recommended by CloudFlare it might be best if you run tests against your own server first, to find out the root of the problem, before to turn on CloudFlare:
https://support.cloudflare.com/hc/en-us/articles/200171936-Error-520-Web-server-is-returning-an-unknown-error
Luckily, you should be able to do just that using your test server.
6. The memory limit is not defined in wp-config.php or wp-settings.php - should I add it or does it default to 40M anyway?
It defaults to 40M unless you don't have 40M available on the server. However, looking at your last post, it doesn't seem like memory is the issue here.
A) Install "Delete Expired Transients" plugin and delete expired transients. (Note I did not do "all transients -- use with caution! because I don't really understand what this does)
Could you give that a try on your test server? Deleting all transients shouldn't cause any more trouble than you already have. You will most likey notice a bump as transients get set again, but at least you'll start with fresh, non-cached data.
Monarch Plugin (Share On Theme123.Net)
This got me curious, as the Monarch plugin was originally developed by Elegant Themes:
https://www.elegantthemes.com/plugins/monarch/
Do you use a forked version of the plugin? If so, I'd recommend using the original instead. While it is a paid plugin, you'll be able to trust the authors of that plugin. I'm not familiar with Theme123.Net, but a quick look at the site shows that they're redistributing paid plugins and themes. What we don't know, however, is what they add to the plugins before to redistribute, and what that plugin may have added to your installation (backdoors, extra scripts, changes to other plugins or your theme, ...). I'd strongly recommend against using plugins and themes from such sources.
It might be best to deactivate and delete that plugin, and then start with a fresh installation of WordPress, where you'll import your existing content with WordPress' built-in importer under Tools > Import. Once you've done, you can download and install each one of your plugins again. It's best not to copy them from your old site, but to download and install them from scratch, to be sure you'll start with a fresh installation.
Let me know how it goes.