I'm currently not in the position to test this but i was wondering if this is possible? I know for a fact that the creator of the mod in which the grindstone is (applied-energistics) in didn't want to make the player able to automate the grindstone but since the autonomous-activator simulates the players clicks I thought it might be a viable option.
Yes. The autonomous activator acts as a fake player, simulating clicks as configured. It works on virtually anything that is activated by right clicking repeatedly, including the Applied Energistics grindstone. Using an Engineering Turtle from ComputerCraft also works. There's nothing in Applied Energistics that prevents it.
Keep in mind, the grindstone is not a fast way to double ores. It costs no power, sure, but it's really slow in comparison to, say, the pulverizer. So early-game, yes, you can definitely automate it, however, I wouldn't want to be in late-game with that kind of a setup.
Yes and no. In 1.6 and previous versions, this was possible. However, from Minecraft 1.7 onwards, this will not work and the crank will drop instead.(Source: here) This is because the Autonomous Activator is a Fake Player and the code checks if it is a fake player. However, there are other options. They are:
Macerator from Industrial Craft
Pulverizer from Thermal Expansion