I use the same 4:1 ratio of brown sugar to salt. I do everything exactly the same way you did, except...I smoke at 170, I pull the salmon out at 135 and loosely tent with foil. I use cherry or maple (don't really care for the taste alder personally, even though it is super popular for salmon). Don't worry about that little mini-stall. That doesn't sound weird to me at all. Your temps will rise really quickly in the beginning, and slow down as it gets closer to your target temp.
As far as the temperature outside, these are so well insulated, I haven't found that it makes a bit of difference to your temperature setting. It will get there and maintain it just fine, no matter how cold it is. Smoking in the winter is one of my favorite things about this smoker! I live in Wisconsin. And I think winter is the best time to do lower temp smoking like hot-smoked salmon, and cold smoking like cold-smoked salmon lox, because the starting point temp of the box is so cold. I smoke all of my lox in the winter.
As far as how dark, I don't think it's the sugar ratio, because that's what I use, and mine doesn't get that dark. A few theories: Alder: I don't have any experience with alder, but maybe it produces a darker color on the surface? (don't know, others can help with that). Smoke duration: I don't think any of my salmon smokes have gone as long as 3-1/2 hours. More like 1.5 to 2.5 hour range. Is your probe accurate? Maybe put it in some boiling water, and see if it registers 212. If your probe is off, your salmon could have smoked longer than it needed to. Combustion: if your wood was too dry, it might have ignited and produced a lot of black smoke, darkening the surface of the fish. Either way, I think your color issue had something to do with the smoke, but it really doesn't look that dark to me, and I'll bet it was still delicious! I'm wondering about that black spot in the middle though. I'm wondering if moisture built up on the top of your smoker, and dripped? Sometimes I get little black droplets on my salmon. I just dab them with a paper towel when I remove from the smoker.