[akanevsky]

1) Is there a way to 'mass-refill' all member's invitations by a set number? Or, possibly give 20% more invites across the board based on registration volume?
I obviously wouldn't want to handpick & manually adjust for random members by hand.

Yes, there is a special allocation tool for that, and it's interface is very similar to that on the "find user" screen. E.g. you can filter users by anything you want.

2) Is it easy to 'upgrade' from your free Invites mod to the commercial one? Should I uninstall this one first? Will the db changes have issues when installing the commercial version b/c of changes the free one may have already made?

You'll need to undo and then redo template modifications, but aside from that it's just a re-upload of files and the product xml, and then a revert of any modified templates.