Maybe I didn't do a good enough job of it, but the point of that was to show that in all likelihood, the intensity of neiji's pain is greater than that of the rest of the clan for a couple of reasons. One is that he has a greater talent than all the others and having such talent amount to nothing is painful. Also, it didn't help that his father was feeding him all this stuff about him being the most talented, and about wishing he could give birth to him as a head family member. Just reiterating enough to try and clarify what I said. Another thing I should have added was that, even though the realizations neiji came to probably aren't unique to him, he came to them at a very young age. I'd guess that that also separates him from the rest of the clan as far as the intensity of his pain. Also, there was a definate class distinction between head and branch members as shown by the relationship between neiji's father and his uncle. Regardless of the love they had for each other, neiji's dad didn't hesitate to attack hinata, and the brother didn't hesitate to activate the seal, and neiji had to witness all of that...

The reason I bothered trying to make a distinction between neiji's experiences and that of the rest of the clan is because you said "the 'dispair' that you had distinguished from sadness shouldnt be any different than what the other branch members feel" and I felt there was a need to do so to get my point across as a result. Anyway, regardless of how it comes about, I personally would rather go through life with hatred and purpose rather than despair and being unable to take my own life (for whatever the reason, even if I'm just afraid to do so)

About the seal not being on everybody, you're right, it would make more sense to have it on everyone, but part of the Hyuuga's hook are class relations and birthright. I'm guessing the seal being on just the branch family is to make that concept more apparent.