I looked through this thread again to double check myself before writing my response.
First of all, it's a nice smoker, you did not go wrong for that price.
And what I'm thinking, from what you have logged so far on your results is that the exhaust is the root of your problem. Especially with that right angle turn in it. It's not letting the air flow freely enough to get a good draft.
With that size firebox, you should have been able to get that smoker up to a easy 350* , even in those temps. Your intake vent is huge, and if your running with the vents at 3/4 open to get to 300*, that's telling me that it's not letting the air out the other end. I'd like to see a good pic of the opening on the left side of the reverse flow plate, just to see how big that opening is, but I'm sure it's at least twice the size of the exhaust.
So why don't you talk to the builder, explain the problems you are having and tell him that you would like to pay him to redo the exhaust. With the right approach, might get him to do it free or next to nothing.
Once you get the exhaust squared away, the trick for cold weather cooking will be to get that baby up to 400* and get all that steel preheated good before bring the temp back down to where you want it. Then cook at a higher temp than you would on a warm day, say 325 or so,