>(2) Given the complexity of the problem and the involvement of hundreds of research groups and dozens of national and international funding agencies, why give a Nobel Prize to just a couple of research group heads for this new understanding?

Because these two were directly responsible for the discovery and the initial understanding of the mechanism of what was going on. The depth and quantity of the resulting research, funding, and all the other streams of new jobs or additional lines of research are follow-on results from the finding, which to me is a metric that highlights the enormity of their achievement and the richly deserved nature of this award.