Methodology Question: How to deal with others having written about similar topic?
There are a number of issues here, including the purpose of your writing and who you see as the audience. 

Don't however, lose sight of the fact that copyright applies to words (etc.) but not to ideas and concepts. History is full of examples of people coming to the same (or quite different) conclusions independently based on the same evidence - especially when that evidence is common knowledge. 

If you are writing for (potential) scholars it is probably better to give a lot of citations so that others can follow up and expand their knowledge of the field. I would prefer end-notes to foot-notes, however, as they are less disruptive of the page flow. 

If you are writing for popular consumption (a la Carl Sagan) you don't need to reference everything and your description of an introduction seems about right. You could, when writing, also have an appendix in which you point to the work of others who have discussed similar themes whether they come to similar conclusions or not. 

However, if you have to err, it is probably better to err (a bit or a lot) on the side of referencing related work that you know about. The fact that you know about it means that it may have had an effect on your own thinking and you should honor that in general. 

I think that when you are coming to different conclusions than others it might be especially useful to you to cite the work of others. Again, this is more important in scientific/professional writing than in the popular press. If nothing else this gives you the aura of fairness and leaves no one with a misconception that you were trying to mislead them. 

So, what is required to say and what is useful to say may not be quite the same.