Where to begin!
Bicester North and Bicester Town are completely separate stations, on completely different routes, with fares set by different operators.
There is no "Bicester Stations" group for ticketing purposes.
With the exception of the Off-Peak Return, fares from Bicester Town are higher than those from Bicester North.
And as for buying a ticket from Kings Sutton, that'd be no more valid from Bicester Town than a ticket from High Wycombe would be, as Kings Sutton is between Oxford and Banbury, on a totally different route to Bicester Town!
It is to be hoped that this issue is resolved at the fares change before Evergreen 3 is finished, or it'll be a right pain for anyone travelling from Marylebone to Bicester if they miss a train!
Cheers,
Barry