I was trying to reconfigure my settings in virtualmin and webmin to solve a problem with outgoing email (which suddenly began bouncing back with MX failure message last week) and in the process changed settings in postfix configuration to the point that nothing at all works.

what are the proper config settings?

mydestination = $myhostname localhost.$mydomain

or

mydestination = $myhostname, localhost.$mydomain, example.com

Also what is the proper syntax for virtual domain mapping?
And for mail aliases?
I've tried several variations.

alternately, is it possible to completely delete postfix from my server via shell (or virtualmin) and start over with a fresh, default configuration?

Last question: Yes. For example, in Debian it would be something like this:

dpkg --purge --force-depends postfix
apt-get install postfix

But if you are using Debian (or Ubuntu) I can highly recommend this as well:

apt-get install etckeeper