It was my misunderstanding. I wasn't aware, until your lorefluff was posted, that Kambria was a vassal of Wayrest until the rebellion.
How about this: Sometime during the Oblivion Crisis, the minor marriage alliance between Camlorn and Kambria took place as the ever-more-desperate Kambria tried to secure help from Camlorn against the Daedra, mostly in getting access to food and weapons. After the Crisis finished, the ruler of Bhoriane was dead, so Count Moorcroft slid in and took over, giving it to his son as his own fief because Marcel had gained the trust and respect of the people of Bhoriane in his skirmishes with the Daedra on the Bhoriane-Kambria border.
Despite the marriage alliance (albeit a fairly weak alliance based on the minor family members involved in the marriage), Kambria was still the vassal of Wayrest, until the Revolt in 4E18 (due to reasons I'll fluff out later in my CS history section and the County synopsis). In the desperate times of the Revolt, Count Moorcroft turned to the weak allaince with Camlorn, something he did not want to do as he knew it would likely mean bending the knee to a new lord, but it was either that or having his county actively taken over by Elysana, whereas he managed to work out a deal with Camlorn that Kambria would be more or less independent except for the regular duties and obligations as a vassal which were kept of medium harshness (if that's cool with you ASAP; sort of works out as Camlorn gains a vassal regardless and weaken Wayrests power hold in the area).
Kambria is saved, Bhoriane starts getting rebuilt by its glorious Baron Marcel, and the County of Kambria (which includes the city of Bhoriane, sorry if this is getting convoluted) is now a vassal of Camlorn, with that earlier minor marriage being the glue for the alliance.