Is the UMIK the default record device? If so it might be accessed by the remote session. If it needs to be shared try selecting the non-EXCL input option that should allow sharing.
MMM is generally used to measure within a relatively small volume, usually the operator is still and just moves their arm. For such a large area it becomes much less meaningful to have a single average response. You could pick spots every couple of metres and do MMM measurements at each of them.
One common cause of low end roll-offs in measurements is Windows doing noise cancellation processing on the input. Mics may have the same signal on both channels resulting in almost perfect cancellation at low frequencies. Turning off the noise cancellation can vary depending on the PC and its...
The problem is that the result gets the window settings from the first measurement, but its left window is too narrow to span to the start of the aligned sum IR. I have fixed it for the next build, in the meantime you can manually increase the left window.
No. Broadly speaking, a more expensive mic is likely to have better high frequency response before calibration and may have slightly lower noise floor. If your mic has a cal file then the first point doesn't apply, and mic noise floor won't be a factor in your room response measurements.
Make the right IR window longer, that should deal with it. If you adjust it by dragging the icon at the top of the Impulse graph you'll get a preview of the effect on the response. You can change the default in the Analysis preferences.