PhD in machine learning in my life? I'm 37; graduating with BA in math (online degree) this summer
Rather than give a long answer to a long set of questions, let me propose a strategy for getting all your answers fairly quickly. Make an appointment to visit a quality university and a professor in machine learning. It might be a place you'd want to go, but it need not be. Ask him/her these questions and try to work out a plan that might lead you to success. 

I'm going to guess that online-only education will be less valued than in-person education, but by how much, I can't say. 

Yes, you can do quality research part time, but it takes a lot longer than when done full time - especially if you don't have collaborative relationships to aid you. 

The other questions (and even these) should be explored face to face with someone who can answer follow up questions and help you devise a plan. 

Note, too, that 37 is still pretty young. Ignore the age factor.