"(in fact, many drum tracks already have several variations ... but we can only use 2 in each song ... it would be nice to be able to use more than 2 variations of the same drum track in the song)"

You can use "F5" key to change Drums/Variations at any bar.

You can save a RT as Performance track and move it to the Audio track.

With the VST Sync working (nearly fully working) you will be able to load a wav player into a vsti slot and drag extra tracks from BB into it.
There is Temper VSTi that you just insert some audio tracks then drag the BB track into it. Still looking for a simple basic vst to do this.

You can slave BB to Reaper with the midi clock, but that's not working quit right at the moment. You just right click on Reaper's play button.