Kounnoasoumb33
1) No, tickets can only be canceled before the start of the ticket validity period, which would be until the day before in your (point-to-point ticket) case.
2) A ticket is NOT a reservation, it is an admission fee! Passenger numbers are NEVER limited by ticket sales, because the vast majority of tickets or travelcards are not bound to a specific service, but usually valid all day on any earlier or later connection. Therefore regular (not saver) tickets are always available in unlimited numbers, and always at the same price.
Regarding such a cable car, if a gondola should be full on y busy day, you simply queue up like everyone else (regardless whether you already have a ticket or buy it on site) and just take the next one. So just allow enough time.
As the entire Rigi mountain happens to be covered by the Rigi Day Ticket and also the SBB Saver Day Pass (not refundable!), you might have a look a those before worrying about a single (relatively expensive) Kaltbad-Weggis ticket.