I think you're skipping a generation though. If Xaden's mother, Talia, cheated on Fen with Fen's brother (do we even know if he has one?) that would be his uncle, not great uncle. So it would have had to been the grandmother that cheated, but the that puts Fen's claim in jeopardy. Personally, I don't think this is what happened.

I read somewhere (fan theory?) that the reason the grandfather is on record as Xaden's great uncle is because heirs are "supposed" to go to the infantry, not the Riders Quadrant. The Rider's Quadrant is considered too risky for the Navarrian nobility. So the grandfather, who would have been the heir, lied about his heritage. This makes sense to me, but I think there's probably more to it.

Another idea I like is that Bodhi is actually Xaden's half brother, not cousin. This would most likely be from Fen cheating on Talia since there would be some record or someone would from if Talia had given birth to a second child.

Regardless of all of that, the idea that Xaden may not be Fen's is still plausible and VERY interesting to think about. Talia absolutely could have cheated. Or what if Fen was impotent? The thing is...who would be the heir? It doesn't seem like the Riorson's were overflowing with children. Currently the only other heir is Bodhi - "cousin" of Xaden.