Yeah. The hyd connections are under the seat. That the second and third plugs were impossible to open with an adjustible wrench while the one in front was easy says a lot.
I still think the trans and hyd are using a common pool of oil. You can check easily after you get one of the other plugs open by shining a flashlight into the front opening and watching the other one.
You could also leave the front drain open to see how much oil comes out. I would do that anyway to be sure there isn't any water, or other contaminants, in the system. If it is a common pool you should get about 8 gallons.
The fact that there is only one dipstick also suggests that there is a common pool. You mentioned that it appeared to be "overfilled". Unlike an engine crankcase, as long as there is room for the oil to expand when it warms up, there is no problem with the level being above the dipstick "full" mark. Your I&T shop manual will tell you what the tolerances are. Mine says I have a +/- of about 3 gallons so that hydrolic add-ons will not drain the system below what is needed for the transmission.