I have tested up to +-30 and it works, but I am not sure about the cap. You can experiment if you want, and let us know about your findings. The mount_effect is a direct addition/subtraction to a unit's attack when fighting against the particular mount type/class. If a negative value is greater its attack value, then I believe the overall attack will be considered 1 by the engine. No, I don't think charge is connected directly to with mount_effect (though they both affect attack while the unit is charging).