Phew, many questions. ;-) I'll try to answer them as thoroughly as possible.
Re: Second Main Microphone: (This is one of the few features that's still under development, so the following description contains preliminary information. The actual implementation might be different.) - The plan is that the user will be able to select an additional microphone(-array) from the same Venue, most likely one that was positioned further away from the stage than the actual Main Mic. This secondary mic will have its own Ambisonics decoding parameters, also known as "Output Format" within MIR. As an exmple: Personally, I have achieved great results from an additional pair of cardoids pointed to the back of the hall, mixed in 10 or 12 dB quieter than the Main Mic.
The IR sets from both mic positions will be taken into account for the positional prerendering of the individual IRs for each signal source on the stage (or within any valid HotSpot area). This means that the CPU consumption of the actual convolutions won't change at all, just the prerendering will take a little bit longer. The amount of RAM used for the IR sets will increase, of course, but our developers have put quite some effort into optimizing that part, so chances are that MIR Pro's demands regarding RAM might be still lower than MIR's (under most circumstances).
Due to this concept, the secondary mic will have an own volume parameter, but this value will be the same for all instruments (like in the real world ;-) ...). Furthermore, it can't have a dedicated output of its own. Offering this feature would double the CPU load.
Re: Audio Inputs: This will put the same strain on CPU and RAM like any other source within MIR, minus the demands that an virtual instrument would have by itself (which might be a considerable difference, of course!), plus a little overhead for routing.
Re: SSD: You are right, MIR's IR-sets have to reside in RAM, but like I said before: MIR Pro will be highly optimized in this regard. IOW: While VI Pro 2 will benefit from SSDs, their positive effect for MIR and MIR Pro will be small.
BTW - for a (more or less) complete preliminary list of MIR Pro's features see also this thread: -> http://community.vsl.co.at/forums/t/28947.aspx
HTH,