Hi Stig,
the reverb length of MIR Pro is not automatable. The reason for that restriction is that every change applied on the underlying impulse response data will need a complete recalculation of all used IR data. Due to this, audible interruptions of the output signal are inevitable.
Regarding your second question: There is an option in MIR's preferences to apply "Natural Volume" to an instrument automatically, which is most likely what you experience in case of the solo violin. You can de-select this option and apply Natural Volume manually, in case you want to use it on individual Icons.
HTH,