So if the default theme is not showing comments either, then something is up with your WordPress. I’m thinking maybe download a fresh WordPress and overwrite the current files in case something was missed.
As for the live preview…that function, sorry to be blunt here, sucks. lol…
I did a look online and appears missing comments form is common, even dating back 6 years. But the cause seems to be random too.
Would it be possible if to email me a login for your admin, or at the very least, a screenshot of your WordPress Settings >> Discussion screen to show your settings?