Ordinary seats Vs First class

Hello!
I'm still debating between ordinary and first class passes. I think that besides the comfort of the Green Seats, the big advantage is that they might not be as crowded as the ordinary ones during the holidays; however...

1. Do we always have to make reservations for Green Seats?

2. How long in advance do we have make reservations during peak season days? Would it be enough time if we make the reservations early am for the late-pm return same day?

3. Can we make reservations online for same day?

4. If we miss a train that has our reservations, can we just take the ordinary train if we hold the 1st class 7day Pass?

Your answers are very helpful! So, thank you!!!
Elsa