It seems as you are mixing up cause and effect. I'm just asking the obvious after some of your recent messages implied that VSL is a greedy company that doesn't care for its customers. ... which is remarkable considering the fact that I could show you literally hundreds of forum messages or eMails praising VSL's support and its user-driven standards.
But anyway - as a sign that at least _we_ aren't distrustful, here comes the reply to your inquiry:
5) A final "blend" reverb is a must, or are MIR instances on the separate channels/groups enough? I'm very conservative in the use of effects, and would be happy if MIR worked as a spatializer, with no need for additional final reverb (if not for effect).
The necessity for an overall "reverb-glue" is not a technical one, but purely based on taste and habits. As long as you like the results you're able to achieve, there's no need to change them.
As soon as you feel that some "algorithmic motion" might add some excitement to your mix, you can use either MIRacle (MIR Pro's Algorithmic Reverb Add-On which is part of the product) or any other reverb engine you like. Personally I would stay away from other IR-based reverbs in that case, though, as the non-linear modulation of algo-based reverbs will fit this task much better.
HTH,
/Dietz - Vienna Symphonic Library