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It is hard to say, but if they have been healthy up until now, then you should check for environmental changes that could be making them sick. It may likely be plant rust too. This article will help with that:
https://www.gardeningknowhow.com/plant-problems/disease/learn-about-plant-rust-disease-and-rust-treatment.htm

Yes, I would trim it all back and then treat it with a fungicide. The white powdery stuff is powdery mildew. Here is more information that will help with treating it: https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

False cypress, dwarf loblolly, thuja and balsam fir will tolerate wet conditions fairly well.

Boxwoods are popular for this. They grow slowly so do not need to be repotted often. You also might try looking at Inkberry. It has a similar look to boxwood. It grows slowly as well, but is typically a little more cold hardy than boxwood and can take the winter better.

This article will help with the appropriate soil mix for your container evergreens: https://www.gardeningknowhow.com/special/containers/proper-soil-mix-for-evergreen-container-plants-and-trees.htm

Most Christmas trees are normally grown in cooler climates. Since it is not accustomed to being outdoors, I would leave it over winter as is and slowly acclimate the tree outdoors in spring, planting it as you would any other tree. The tree should go into dormancy and can be fertilized following its plantation outdoors. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/trees/christmas-trees/plant-your-christmas-tree.htm

When pulling a limb or trunk to (un)bend, make sure the items you use will not "girdle" the bark. I always suggest that people use a legnth of "dead garden hose" with a sturdy rope or wire run through it. The hose will keep the rope or wire from girdling (which will kill) the plant. Use a metal T-post from a farm or tractor supply store to attach the rope or wire. Use a 6' or 8' post, the 5' will not be sturdy enough.