Q1 yes Q2 yes The Tempest can be configured with HT by-pass although it may cost more; ask Klaus. Remote volume is +$150 on top of the price of the tempest.
The upgrade to mono's is simply the cost of another stratos w/cap upgrade plus the cost for you to send your amp back to odyssey and for your new mono badboy's to return.
Is it worthwhile? This depends on a few things. If you have very inefficient spk's, highly reactive spk's or a combination of both which is even worse, the difference will be significant. If your spk's are a nice even impedance and 90db then the difference will be obvious but not as profound. In either case you still gain because you can place your mono's right next to the spk's with very short cable's, you have optimal separation, greater headroom allowing you to stay in class A possibly all the way: unless your into thrash metal of-course! More control, actually more of everything. Just upgrade to Extreme's and be done with it.