Hey, thanks for trying all those things! If I can just follow up on two of them...
crystalenka try setting Indigo's reverse proxy ports to something other than 80/443 eg 81/444 (that can be done in Indigo's prefs/settings) and try accessing the site at eg https://mysite.test:444
Just to confirm, you have tried changing the reverse proxy ports in the app preferences? This is different than the ports chosen or allocated in the Nginx or Apache site configuration.
That said, I don't think it'll help because the other info you supplied suggests the domain isn't even registered with your DNS.
try using a completely different domain name, one you've never used in MAMP Pro
Would you mind trying a totally new domain anyway? Doing this will ensure Indigo's Dnsmasq code should run afresh. Even just something silly like
If that also fails, I will take a look at the logging around the Dnsmasq calls and see if it will be useful to read your logs, and/or what the next tests should be.