Filling ATF up to filler plug
 

We have a 2006 Saturn Vue with a 2.2 Liter, Automatic, 2 wheel drive. PROBLEM is the rear side plate of transmission has a sizable leak and the repair costs are more than the car value.
QUESTION: Is tightening this side plate an option with the limited accessibility??
QUESTION: If I continue to use vehicle I will never know how full or empty it is as it wasn't built with a dipstick.SO could I just occasionally top off ATF at the filler plug to not be concerned about running low??

It would be nice if a dipstick was available to fit where the plug is.

No dipstick by design.

As long as you check the level with the vehicle on a level surface, the vehicle itself level (tire off, jack adjusted to support vehicle as level), with the transmission fluid at operating temperature, engine running, in park, the fill plug should be just dribbling fluid when it is full..
You must do this at operating temperature for the transmission due to the fact that the fluid expands significantly as it heats up. Doing it on cold fluid will result in an overfilled transmission.

What exactly is leaking? Is it possibly just the replacement of a deal between the side plate and the rest of the transmission?

If the plate is loose because its fasteners have backed out, tightening might help it seal.

If there is a seal present and the seal is bad, tightening it will likely only make it leak worse.

Post a picture if you would like to and we can have a looksie...... I am not a transmission person but we will see what we can do