How to find the paper that initializes a new field
THe Wikipedia article on this topic is reasonably good and has some references.  I would look at that article and the references within.
Try tracing back "references of references" of reviews or important papers.  (You don't have to read in depth.)  But you may figure out the better papers.  May also find those that have a perspective on (3).
In general, I think "machine learning" is a misnomer and a buzzword.  Talking about it as some organic "thing" is not correct.  You should see it as a bit of a fad (especially the use of the term) and within the broader fields of statistics, algorithms, optimization, etc.  It's not like discovering X-rays or something.