It depends whether you lost track of a scene or you are still oriented in what's actually going on. It would not bother me if I zone out for a moment because I think it happen even unconsciously - sometimes you may be lost for a moment, but your brain follow the story anyway. Nobody has a full picture - it's impossible for human being to absorb every detail on the first attempt.

I've switched to reading in English quite recently, it took me a lot of effort to reach acceptable fluency. Since that time I don't track my activity, here's why. When I was reading in my native language I barely could keep up with my backlog so in most cases I was in a constant rush. Now I need to be more focused hence I enjoy reading itself and don't care about how many pages I've read, what series, which author etc. It's just more mind-healthy

Depends on your level. I personally have thrown myself in the deep end with reading starting from complex fantasy stories and have learned how to distinguish between words really useful and those I could leave out. I think this is the key to get over with a fact that it's impossible to pay attention to every detail and not get bore to death. Reading should be enjoyable and as long as you understand let's say 80% of each page, your are fluent-reader - as you come across some unkown expressions you will finally absorb them. Plenty of them are really unnecesary for daily life and even natives deem them weird or quirky.