Hi Moirabee
Yes, point-to-point tickets are valid on every train until 05:00 am the following day. It is not necessary to buy them in advance, the price is the same at ticket offices or vending machines after your arrival. In the event of major disruptions, your flight could arrive the next day and your ticket would no longer be valid.
I heard that free passes are no more available, you must buy a ticket from Genève-Aéroport.
Kind regards
Danhiel