Will brushless fan speed depend on amperage?
Fans used in this manner, where high heat is involved, are going to fail quickly unless design techniques keep the temperature of the motor and its bearings within the ratings specified by the fan manufacturer. 

So you need to be sure to place fans on the upstream side of the combustion area so that they push the air into the stove as opposed to trying to suck air from the combustion area. Secondly the fans need to be mounted on some type of ducting that moves them far enough away from the combustion chamber that so that they can remain cool enough to survive for their expected normal lifetime.

I suspect that your current failures are bearing problems resulting in the motors seizing up and thus being unable to turn. You should research motor bearing types and find fans with bearings specifically designed for higher temperature operation.