I have a cycle reservation ticket which I bought on Trainline for a XC train from Banbury to Darlington and then back from Durham to Banbury via Birmingham New Street. So, in theory no problem. But on XC's own site there did not appear to be any cycle reservations for one journey. a.) how is this possible?
It's possible there are no more cycle spaces available (more likely if the date of travel is soon), or that cycle reservations haven't yet been released (more likely if it's a long time until the date of travel).
XC only have 2 reserveable cycle spaces per train, and you're making a fairly long journey. So it is plausible that both spaces might already have been reserved.
and b.) is my reservation any kind of guarantee
It gives you the right to use one of the two reserved spaces, but in my experience XC bike reservations are more notional than real - there's no display on the train to show you have reserved the space, just a sign. People often use the reserved spaces even if they don't have a reservation.
Fortunately, there are three cycle spaces (one is unreservable) as well as a large luggage storage area immediately adjacent which is rarely full, although sometimes people are forced to sit there due to the lack of seats!
c.) what happens if my train is cancelled or replaced by a bus etc (it is not scheduled to be).
If it's cancelled and there is a later train you can catch, you could travel on that and hope that the unreserved space is vacant. If replacement buses are in operation, you will likely have to take a different route as non-folding bikes aren't usually allowed on replacement buses.
In either case, I would suggest speaking to station staff or XC via social media to see what they suggest.
