sorry but .. no....

If I were to uninstall 1.6 and clean install .91, would all the posts / comments be loaded onto that install because they're on the mySQL DB right?

Thanks by the way

When you upgrade to 1.6 your database is "reconditioned", meaning that it is changed to fit the new 1.6 architecture. If you "downgrade" to .9.1 the installer will not know how to reverse-condition your database to fit back with the .9.1 architecture.

That's my guess as to why Topanga said it can't be done.

exactly.
It is not possible automaticaly.
Maby-maby-maby

you can manually upload your posts and comments, but for the rest, you will have to do it all manualy.
It will be a hell of a job with no garantie of success
that's why I said no