If we have a reservation for 7 and only 5 arrive, will that be a problem?
I can't speak from experience, but according to this page only one person needs to be present to consider a dining reservation fulfilled. I am hoping that is true for my upcoming trip.
Edit: You can find what I'm talking about in the 5th paragraph on that linked page.
It is true only one person needs to arrive for the reservation. I am traveling on my next trip at the same time my father will be in the world. I have made all my ADRs adding him and his wife because I can show up with less, but more could be a problem.
I've made one for twelve and shown up with two. No issue.
Should not be an issue. I've made for 2 and shown up with 1 before. Plus a 7 to 5 reservation change shouldn't affect the size of the table required for your party, which can sometimes be an issue if you're time crunched for a fp+ or other reservation and the restaurant wants to seat you at a smaller or larger table than planned.