Sorry, for some reason I missed your question earlier. I think the moisture on the bottom board is condensation. You will need to do something about this, or you risk losing your bees over winter. Cold dry bees do fine, cold wet bees often don’t survive. I think a quilt box on top would be very helpful. You can either build one or buy one. Building one is pretty simple, and Rusty Burlew has a nice set of instructions here:
If you would rather have one ready made, you can get one from Bee Thinking:
https://www.beethinking.com/collections/parts-accessories/products/langstroth-quilt-box?variant=20610688708
I think you did the right thing to retreat for varroa. They do continue to drop for a month or so after treating in my experience. Obviously you don’t want to over-treat, as even Oxalic Acid can be toxic for bees. Your slider gives an interesting clue as to where the cluster is concentrating! 
(More debris in the upper left corner of the photo)