***"What happened to the last quart?"*** Well, a tiny bit may have evaporated, maybe a couple or 3 tablespoons. The other part you don't recover is the volume that is trapped inside the pump, the motor, the valving circuits, and the rest is just clinging to surfaces or lying in pools where it can't drain from.
***"5 full quarts in and it went about an inch over the 'full' line. Is this a problem?"***
It will only be a problem if the oil begins to be expelled from the breather, vent, or tube.......due to heat from hard useage. One of the things that can happen in hydrostats is the tendency for oil to "foam" in use due to inevitable air entrainment. Therefore, antifoaming agents are included in the additive package blended in the formula for the oil. Over time, the anti foam stuff begins to deteriorate and can no longer cause the air to "dis-entrain", or "release from the oil". When this occurs, foaming can cause hot oil to be expelled from any breather provision on the trans. My advice: Run it as is, until you want to change the oil again, or until the oil begins to escape from the vent............whichever occurs first. Air entrainment is insidious in that the trapped, entrained air actually supports "micro-combustion" or oxydation of some of the oil molecules under conditions of extreme pressure in the pump and/or motor. The pressure can be great enough for oxygen molecules in the entrained air to actually support combustion of oil molecules. But you don't need to worry about your tranny catching fire, the "mix" is much too rich for that to happen. And you thought you already had enough to worry about.

I have a K71? on my GT235 and have always run into the same issue. Since you can't bleed the transmission, it takes some time for it to accommodate more fluid, usually two to three uses. Only fill to the half way mark, cut and the next time you should then have to fill again to the half way mark. If you've filled too high, it will expand and overflow.

Thanks for the prompt answer.