"I plan to interview polyglots, linguists, teachers, expats, successful students, interpreters, lexicographers, etc.."It's definitely a good idea. There is a lot to be learned from polyglots, but sadly, there seems to be very little that can be learned from the internet polyglot community.
The problem is simple: most are happy to tell you what they do, but a great many are not happy to let you find out what they do. You can listen to what they say, but you can rarely probe or challenge it, which is a nuisance, because very few of us are ever fully aware of what we do.
Stepping away from the world of languages, consider this story I was told in my university days. There was a cheese factory near Edinburgh, and one of their employees specialised in determining when the cheese was ripe enough for packaging and selling. He was nearing retirement, and the company hoped to make a machine to do his job rather than training a replacement, so they called in experts on AI and machine learning to try to create a robot for the purpose. They asked the man how he determined when the cheese was ready, and he told them he prodded it with his thumb, and he knew if it was ready based on how springy it was. They set up a machine that bounced a little thumb-sized probe on the cheeses and measured their springiness. The problem was, there was absolutely no correlation between the measurable springiness of the cheese and the expert's judgement on whether it was ready or not. Eventually they discovered that in prodding the cheese, the man had been breaking the surface, which released a scent that he subconsciously detected. As computers can't smell (yet), the project was abandoned, and an apprentice hired.
Going back to language, for years I derided those who talked about "shadowing" other people (ie repeating audio books or films verbatim), thinking it was a passive process that wasn't truly "linguistic". I didn't shadow, anyway. But then I remembered that I used to shadow DVDs in Spanish. Not religiously, not obsessively, but from time to time. I did it, and I can't say it didn't help me -- it might have, it might not have. I can say I believe that it is of minor utility at best, but I have to be careful not to say what it "is" or "isn't".
If I can enter calmly into a discussion with other people, I can often be made to realise that I have fixated on one thing and completely ignored something else, and I can in turn make them realise that they have done the same thing. The end goal of such a discussion shouldn't be to walk away with a better understanding of each other's techniques but for each to walk away with a better understanding of their own. When we enter such a conversation with the view that we are correct and the goal of convincing everyone else that we are, we add no value.
Interviews can be a starting point, but only if the interviewee is engaged in discussion rather than merely lecturing those he sees as less informed.