I have one of these cases but I don't use it as much as I thought I would. It does have the opening in the back for the on off switch. I don't remember if it has the access for the MR-3. I'm 99% sure it does. I have an MR-3. I just moved to a new house an have not located the case in all the boxes to look for sure. I used to use the lower part of an everyready case on every camera for protection. I've worn out some in the past.
I do a different type of photography now, so I don't use them as often. I discovered when I use the motor drive I tend to go through more film and the case slows down film changing times. If I was outdoors a lot or in dusty climates I would use this case more. Hope this helps.