I still don’t know what the heck caused problems, except that I set up MX records pointing to MXROUTE services. Just MX and… I forget now which of SPF/DMARC/DKIM. But none of the A or NS records, for example, for which I’d posted screenshots…
I don’t know why changing the MX records caused the domain to eventually disappear from external DNS providers, i.e. no longer able to be looked up…
At some point, I’ll experiment with a less important domain than my primary business domain, I suppose. I don’t know why messing with the MX records would cause the domain to stop working, but that’s apparently the cause as it happened three times - I deleted and recreated the virtual server, and it worked - until I messed with the MX records, then as caching ran out, the domain stopped being accessible from browsers or global DNS propagation checks.
Mostly coming back to add this in case anyone else runs across this problem and winds up in this thread.