You indicated that you have severe winters, which means that the ground temperature on the other side of the basement walls and floor is probably in the 50 to 55 degree range. That being the case, the last thing you want to do in the summer is open the basement windows and let in humid air that has a dew point higher than 50 degrees. The result is the moisture in the air will condense out on the basement walls/floor. I would recommend that you keep the windows closed. Next, taking 3 days for the dehumidifier to fill up is way too long IF it is running all the time. It is possible that either it needs to be turned up or possibly replaced with a higher capacity model? I have lived in houses in which the dehumidifier had to be emptied twice a day, and that was without any visible moisture in the basement. If you have central air in the house, you might consider opening or adding a return air vent in the basement area to help with the dehumifcation process.
Hope This Helps,
P.S. Sorry if your first visit to this site was a negative one. I was only trying to make the point that one should remodel to what they like, and not try to please some future buyer. I think you will find the "help" on this site by and large to be concerned and competent.