Sounds reasonable. Of course, this would be an alt-Bannockburn which could have a different outcome. If the English are defeated I doubt very much Bob Snr is gonna necessarily want an English bride - Hell, he might see it as a sort of vindication and start looking to the continent/France for a bride. If the Scots lose, however, then the English can force Bob Jnr and Ms de Clare's marriage (and just BTW, the de Clares descended from an illegitimate half-brother (or somesuch) of William the Conqueror, so they were about as close to princes du sang of the early Plantagenets/house of Normandy, until King John's two boys, that one could get).