If you set your volume faders all to the the same level, turn on Natural Volume, and use only velocity (and instrument-positioning in the MIR room) to control loudness, your will get combinations of timbres limited to the combinations which are possible in the real world.
No, that is not correct, because it ignores the differing dynamic range of instruments. There is probably one dynamic at which this is true, but as I have no idea as to how the Natural Volumes were calculated, I have no idea which dynamic it would be.
I'm not understanding your point. Not disagreeing with you, but I don't understand what you mean.
To use Mural as an example, if I set my volume faders to the same level and use only velocity to control loudness, I get only the timbre-combinations which are possible in the real world. I get this despite the differing dynamic range of instruments. Or are you suggesting Mural doesn't work this way either?
I have no idea how Mural works so I can't comment on that. However, I do know that the dynamic ranges of VI are not really accurate. For example the dynamic range of a Flute is far less than say a Trumpet, but in the VI player the difference is less severe than it should be. Maybe this is something that the VSL guys could comment on and possibly give figures as to how the normalisation (for want of a better word) has been achieved.
The other thing to remember is that certain instruments react with their surroundings far more than others, and the convolution IRs cannot reflect this, as they are based on sounds coming out of a speaker. In fact you'll find that even if one was to broadcast a real performance from an anechoic chamber into an acoustic via a speaker, it still wouldn't sound the same as having a player there.
There is science to be gleaned from all of this, but I think that it is far to complicated for any company to solve in the short term, and what we are left with is generalities. I agree that the science of it ought to be nailed down as far as possible, but also understand that there are too many variables for any definitive answer.
DG