Hello. I have a 27' round AG and I put a solid cover over it every year. I drain the water using the return line. I diconnect it from the filter and just let water drain out of the pool. This leaves the water level just below the skimmer. Any rain that comes before freezing temps do will lower it a couple more inches (by way of the open return line). From what you described it sounds as if the water has no where to go but over the top of the pool. I would just leave the return line open. Any water that drains out of it does so very slowly. If you are concerned about water dropping too low you could always add water during the winter. You'll know when it's back up to the bottom of the return line when you see water coming out of the return. Of course this assumes that your return is a bit lower than your skimmer (mine is only slightly, but this is all that's needed).

My problem has been keeping the cover on during 40+ mph winds. Last year I used some milk jugs but it wasn't enough. This year every colum gets one.