2)I've noticed that a lot of films shot on Arri Alexa that get released in Imax get released on the imax digital format only, but on most red movies they get released on film imax as well as digital. Is that because Red has a much higher resolution than Alexa?
2) On the surface, the answer could be yes. A 4K image could print out to IMAX film with better quality than a 2K image. I think the reasons are a lot more difficult to guess; mostly having to do with distributors and studios.
The actual ISO equivalent of the M-X sensor with regard to responsiveness and targeted color science is 320 and 800 respectively. ISO is metadata, though, so there really is no higher ISO than the native equivalent of the sensor regardless of what options are available in-camera. Choosing a higher ISO setting is like cranking up a post gain control.