Good questions and good points.
The installer maybe should be changed to show all DB driver options and show a warning if the driver is not installed. Please, create a GH issue for it.
I googled around and found this tool to convert PostgreSQL dump into MySql: http://www.lightbox.ca/pg2mysql.php
Do you have some statistics supporting your statement? Maybe if we’d show them to the Mautic users, they’d use Postgres more often. Right now it is 0.7% which is quite small number to support it.