Sorry you had such a bad experience...
As I said before your trip, cheap deals tend to be for return tickets but not for singles. Your original plan was for a return train ticket to York for which a "walk-on" fare of GBP 64.00 sounds quite reasonable to me. I don't know how much you would have saved on the return fare by booking ahead, but I doubt it would have been more than about 25%. By choosing to travel one way you ended up paying the full fare, and as you said it would have been cheaper to go by car.
As for your specific questions:
1) It depends on the type of ticket, but on the cheapest deals the tickets are usually only valid on a specific train (just like cheap airline tickets), and if you've missed your train you cannot travel on another one even if you stand for the whole journey. If you want a ticket that is valid on any train you pay more. (NB The more expensive fares generally come without reservations, and you either take pot luck or pay extra for a reservation - so the ticket and the reservation aren't linked)
2) I doubt it. It may well work for Gatwick (because fares to London airports are so extortionate), it may occasionally work for other destinations if a train company is running a "special" to a specific place, but it's the exception rather than the rule, and I wouldn't count on it.
[This message has been edited by Aviatrix (edited 12-14-2002).]