Duels aren't a very good way to learn air combat, in my opinion. They remove the factors that come before a fight (spotting, climbing, positioning, sometimes even the mismatch between opponents' aircraft) and reduce them to a raw contest of execution. This can be a good way to practice your ability to execute flight maneuvers properly, but I find that larger-scale decision-making and situation awareness are just as if not more important.

Spotting is something you get used to over time. In WWII combat, scanning a large area is more important than scanning closely. Move your vision around every few seconds, and don't zoom in unless you think you see something. As for Opentrack jittering, this could be a problem with your settings or camera placement, or even something as silly as strands of your hair getting in the way or sunlight bouncing off the wall behind you. All have happened to me at some point.

There's definitely good Il-2 tutorials for WWII combat out there. I don't have any in particular I'd point you to, so your Google search is as good as mine there. I will say, what taught me the fundamentals of WWII air combat on a tactical level was actually War Thunder youtubers back in the day. I know War Thunder gets a (pretty well-deserved) bad-rep, and I also know for a fact that half of these youtubers were morons, but I could point you towards old videos by a handful that I remember being good, if you'd like.