I'm speaking as Canadian Marriott cardholder (we get 1 Cat 1-4 free night + 50K points here)

You'll be fine to use the free night whenever
The Free night is in the Marriott account, nothing to do with Chase

As for booking for 3rd party/non cardholder, I think as long as your name is somewhere on the reservation and you can show ID at check-in, you're okay too
You can pay incidentals with non-Chase CC also