I don't ride in the winter so I can't answer some of your questions. I put Proofhide on the top and bottom leather, that appears to protect the underside quite well. I don't trust waterproof covers, I put a plastic bag under the waterproof covers so that if and when the cover leaks the plastic bag protects the leather. In other words, I use the cover to protect the plastic bag that I rely on for waterproofness.

If I did ride in the winter, I might be tempted to put a cheap plastic saddle on the bike if I was going to go riding while it was snowing.

I hope you are not one of those people that ride down the center of residential streets in one of two tire tracks at about 6 mph on a snowy day preventing traffic from getting past. I hate those people, they do not realize how much road rage against cyclists they are causing.