They make special clamps to hang both vertical and horizontal runs of pvc. If the big box stores don't have them, go to a plumbing supply store.
When going through the roof, if anything, you want to increase the diameter of the vent, not decrease it. In really cold weather, humidity will condense and freeze which can close off a vent that is too small.
Freezing traps can be a problem if it is cold and drafty under the cabin. For times when it isn't being heated, pour some RV antifreeze into the traps. The drain lines, other than the traps don't need to be insulated. When you do the water pipes, be very careful so that you can drain them...no dips or perfectly horizontal runs so you can get them to drain well.