The mAh rating of a battery has nothing to do with it's ability to blow a motor.
mA is a measure of current, you can think of it like speed/velocity in miles per hour
mAh is a measure of current * time, similar to how miles per hour * time would be miles!
If your car can go 400 miles on one tank of gas, that is not the same as saying it can go 400 miles/hour.
YES, the 2 9V's in series will have a shorter battery life
YES, they will see more current draw due to higher voltage
YES, they will make the motor spin faster
YES, they will burn out the motor faster (but not because of mAh ratings)
V=I*R should give you a good estimation of how much shorter the battery life
will be. R will drop out, voltage is 9V vs. 4.5V, so current draw is 2x. Electric
charge is (for example) 1000mAh vs. 2850mAh, so about 35% of the original.
So you have 35% the available power, and it is drawn twice as fast. That
means you will get approx. 17.5% the battery life you would from 3 AA's!
Maybe I am misunderstanding what you're saying. I think we're both on the
same track, but some of your statements do not make sense in to me. I hope
my explanation is clear and (hopefully) error-free