Need help uninstalling Mountain Lion and going back to Lion.

I would be very interested in the answer to this question - MATLAB's integration into Mountain Lion is just god-awful (10s delays between clicking the cursor and it registering on screen are normal on my top-of-the-line Retina with extra RAM)

You can't "uninstall" an Operating System. Unfortunately, since you don't have a current backup for Lion and you don't have the installer for Lion you're stuck.

That is really bad news, Apple and MATLAB should either provide a fix ASAP or provide a warning that MATLAB exeriences issues with OSX 10.8, because I'm finding this combination unworkable!

You mean the producers of MATLAB should provide a fix ASAP, as it is their software and is their responsibility to make sure the software is compatible with the latest OS.

I guess, whoever, but I wish Apple kept X11 compatibility in 10.8X, as MATLAB worked just fine using that

Try this Apple KB article: http://support.apple.com/kb/HT5293?viewlocale=en_US&locale=en_US

Did you by chance create a USB thumb drive with the Lion Recovery HD on it? Hopefully you did. If so, you can boot from that thumb drive and you will see the Lion logo for the Reinstall Mac OS X selection on the Utilities menu.