I agree with most of your points and would be interested in answers to your questions. For the very last question about lower-income children and age, I think the article is suggesting that because richer parents are red-shirting, state governments are raising the minimum age for kindergarten. Thus lower-income children who would start earlier are forced to start later.

State government have raised the minimum age for kindergarten in the past 20 years. It used to be that everyone born in a certain year went to kindergarten, not it is almost universal that children must be 5 by Sept. I am not convinced that red-shirting caused this change. I do, however, know people who are now wondering if they should send their child with a summer birthday to school when they have only just turned five. This is not just because they will be the youngest, it is also because kindergarten is more academic than it used to be.