The dark spots could be frostbite, that bled a bit, and the blood got smeared on your feeder when he ate.
I have a metal feeder, and I don't think any of my birds have ever gotten stuck on it.
But........
If you think that your birds are getting stuck, I would get the silicone spray used on...
I have two Comets.... They are cold hardy, and good birds, the problem is just that they have such giant combs, so the comb might get some frost bite on it.
Best thing to stop frost bite is LOWER your humidity.
So, make sure you have lots of venting in your coop, just make sure there isn't any...
I would only cover them if there is a strong wind that blows through there.
But, as long as you leave some open for ventilation, it is OK if you decide to close some off.
galaxy,
I would think that what you are doing right now would work for the winter temperature wise.....but I am not sure how it will work snow wise....not sure how much you get though.
If the snow isn't an issue, I would not use any heat lamps, but I would put in a wind break in the outside...
What to do to winterize:
If the wind is blocked, they can be tempted to go outside.
Going outside is good for them, and will actually help them stay healthy.
Sometimes they are picky about having to walk on snow, but you can try to keep the snow out of the run, or toss dried grass clippings...