Yes, that about sums it up! And yes, that works as a rule of thumb.
Az'Lean is being honest there, he respects your decision either way and it doesn't change his relationship stat. However, he will take note of whether you have similar views to him or not (this is one of those hiddens stats I mentioned earlier), and let's say if you disagree with him multiple times, there might be a threshold where it does negatively impact the relationship.
That's difficult to answer. In theory, you can pursue someone no matter what side you are on. So you can side with the fae and still go for Az'Lean, but meeting the requirements is a bit harder (since you're relationship might not rise as high if you say and do a lot of things he doesn't like). Also, it might not turn out well from a story perspective. You can't secretly work against him and expect your relationship to go well, haha. That doesn't have to mean a break-up though. You will get an ending with him, it might just not be a happy one.