It does not deal a blow to anything. The Prophets (s) were rejected by entire nations after clear proof, and sometimes by their own family members. Why is it hard to fathom that people can be really stubborn in their disbelief? Anyone who reads the stories of the Prophets (s) in the Qur'an can see this is the case.
I do not agree with your comparison. What you are referring to was outright rejection of a message, in its entirety. In the case of Imamat, you are saying that there were people, even offspring of the "infallibles" (ra), who accepted Islam but rejected or were oblivious to a (supposed) usool of it. They accepted all the "hard" stuff - the dos and the don'ts - but relapsed when it came to a rather simple belief. That can mean one of two things: either they were not good Muslims or the belief they relapsed on is not present in Islam, starting with the Qur'an. No Shia has been able to provide one unequivocal, unambiguous verse to support Imamat so it is quite clear that it is the latter.
As for no authentic evidence of Abu Talib (ra) being a Muslim, that is incorrect. Our sahih narrations mention that he was a Muslim. Now if you believe he is a kafir based on your hadiths, that is your choice. I stick to my books, you stick to yours.
We have seen numerous examples where Shias on this site, or even on ShiaChat, reject authentic Shia narrations based on their own logic. In fact, there are authentic narrations that contradict each other. Hence, the quality of your books is well-known though I have tremendous respect for all your narrations. One of my teachers used to teach us that you should respect a narration if it bears the name of the Prophet (saw), even if it is weak. You do not have to accept it; just don't mock it. Having said that, don't you think it is quite comical when you see the fervor with which Shias argue with the rest of Muslims to prove the faith of Abu Talib (may Allah deal justly with him) when they condemn Muslim (having accepted Islam) members of the bloodline of the Prophet (saw) to Hell for differing on a trivial issue, such as the next Imam?
Truth be told, I have more respect and affection for Abu Talib (someone who did not accept Islam) for aiding the Prophet (saw) than you have for the Companions (ra) of the Prophet (saw), all of whom accepted Islam. Let us not even get to the children of Imam Ali (ra); I think you already got my point!
As for what I said about "adalat awlaad Ali" - you nailed it. I do believe they are bad if they reject the correct Imam. As for them being unjust, I did not claim anything of thay sort. I didn't even bring that word up.
"Adala" means justice. To not believe in "adalat awlaad Ali" is to believe that they are not just, therefore, unjust.
As for the rest of your comment, that is an entirely different argument. If you want to discuss Mu'awiyah and Hasan (as) and the Imams (as) you can open a different thread on that.
True; an entirely different argument. However, it amazes me that such commonly known events don't prop up in your head when you consider them to be "infallible". Much like what I said earlier regarding your narrations; two authentic narrations negating, or standing opposite, each other. This is similar; two "infallibles" (ra) contradicting each other in action. The predicament (for our Shia brothers) is that the Prophet (saw) praised the one who made peace with Muawiya.....for no other reason but because he made
peace.