Turning off servos with a MOSFET
I don't see how that would happen. The control wire is an input, it probably has some clamping protection diodes somewhere, but I don't think it can source (or sink) an appreciable amount of current.
If you want to be extra sure set the Arduino pin as an input when you turn off the mosfet: little or no current can flow in a microcontroller input, so there would be no path for current that would come from \$V_{cc}\$ to go anywhere.