The fluid will start to foam and leak through the filler tube were your dipstick is located. It may also cause seals and gaskets to leak. If accidentally overfilled, drain some by removing the plug.
a big mess.
Possibility of fluid foaming and eventual transmission failure
overfill vent on top usually blows excess out of trans.
yes
Overfill plug on an A/T? Never heard of one. You add fluid at the same place you check it. Do not overfill.
Depends upon the model of transmission. On some, it is no big deal, on others, the fluid will foam and possibly shoot out of the filler tube. Others, it will cause the transmission to slip and possible overheat.
Yes
Yes
Through the dipstick tube--Do Not overfill
Yes, of course. A transmission can only hold so much transmission fluid.
Yes
Usually through the dipstick tube - Do Not Overfill