1) Ableton Live allows you to add virtually limitless numbers of VSTs to any channel, or multiple channels, or the master. The major limitation will be your computer's processing power.
2) See #1 for the first part of the question, in terms of swapping, you can have the VST windows opened or closed, but all instances will be controlled by the RMX hardware because it is pre-mapped for the MIDI control. You can change the MIDI channel in the VST window for each instance, but then you'd require other MIDI hardware to control it. What you could do is simply click the "Plug&Play" button in each RMX VST window and thus disable the incoming control for that instance.
3) Yes - the effects will be in time.with Live's master clock.