I see, so the general consensus is that the Byakugan was the progenitor for all Dojutsu. If so that could explain how Madara could tell that the real Naruto was not there and that Obito was fighting clones, but at the same time I find it quite odd that Madara would bother to revert back to the Sharingan from the Rinnegan to cast Genjutsu on Raikage and was not shown reverting to Byakugan to examine Naruto. I do rather like the idea though, it would be interesting if Madara could use all three doujutsu seeing as how he has the knowledge of the tablet left by the 1st Uchiha.
I think it's simply more a show of clan arrogance rather than becoming so inbred that it's impossible to marry outside of the clan. Keeping the bloodline pure has been done often throughout history because they thought it keeps their bloodline strong when in actuality the lack of diversity weakens it. It probably all boils down to a simple matter of control. If a hyuuga were to marry into other clans they wouldn't be able to control them as if they were their own. The mark on Neji's forehead is a testament to how despotic the Hyuuga clan is.
I often wonder if the SO6P's sons were birthed naturally, and weren't simply created from Izanagi. Distributing specific traits amongst your children seems like it wouldn't occur naturally, and the only other way I could see it happening is if he could somehow manipulate the birthing process and designate which traits of his they would receive while they were still in the womb.