I can install and run Linux on a Mac can't I?
Barely. Linux does not run well on Macs from the last few years. You'll most likely run into major issues with wifi, sleep/hibernate, bluetooth, the trackpad, the touch bar and more.
the primary software that is used by nearly every employee for 80+% of their job requires IE to be the default browser
I would quit if I were you.
You have a reasonably smooth upgrade path to PostgreSQL, in fact if you are using database abstraction as you should, it's a config option.
Um, no, unless you use such an incredibly small subset of SQL that you are not using the database for more than storing and retrieving your application data as-is. Depending on your code, it may be just minor adjustments or it may require a full application architecture overhaul to support a second database. Realistically, no in-house application ever changes database engine without it being a part of a major rewrite and rearchitecturing anyway. If you develop for PostgreSQL, take full advantage of its fantastic feature set. Don't restrict yourself and your coworkers to the 5% of it that it shares with MySQL just to be able to switch with a config option.
"Take that, you hostile sons-of-bitches!" -- James Coburn, in the finale of _The_President's_Analyst_
