Quran is clear, that those send by God (Mursaleen) have to be accepted, and that submission towards the most recent chosen choice of God is a requirement and that it's not acceptable to not respond to God with respect to his guidance and caller.
السلام عليكم ورحمة الله وبركاته
شفاك الله وأحسن الله إليك
As far as the Qur'an being clear that we must follow the Prophets and Messengers Allah سبحانه وتعالى sends, then of course this is true and no two Muslims would disagree with such a claim. However, if there was anything past that that was obvious, then there is no way logically that none of the experts of the Qur'an would ever reach this conclusion. Lets face it, not a single 12er scholar around today can recite the Qur'an properly (except maybe Kamal al-Haydari), let alone considered experts. Therefore, logically, anything that is understood by the 12er clergy that hasn't been understood by the Muslims (Sunni, Najdi, Zaydi, Ashari, Maturidi, Mutazili, liberal, conservative etc), is rejected and considered delusions by unqualified people.
The Quran is clear, if Mohammad has no chosen successors or leaders after him, it's disbelief to claim it. In fact, following a leader not appointed by God by an Shia sect is condemned as disbelief per Twelver Shiite hadiths as well.
So Shiites should be fine accepting we are disbelievers from the sunni perspective.
Unfortunately, as is usually the case with your posts, you are unable to articulate yourself in a way that gets your point across without simultaneously contradicting the point your are trying to make. This is because you believe, then try to find a way to justify it, instead of studying the Deen, then forming your beliefs. The reality is, you:
a) Recognize that the 12ers make takfeer of the rest of the Muslims,
b) recognize that the Muslims don't make takfeer of the 12ers, except the Khawarji and some of the Najdis,
c) so you reached the conclusion that the 12ers and the Khawarij/Najdis are really the only people that make takfeer of the rest of the Ummah.
So what did you do? You decided to write this post, in order to find a way to justify takfeer of the 12ers from the Mainstream Muslims' perspective. الحمدلله, the Muslims, from the ibn Taymiyyah's to the Ali Gomah's don't make takfeer of the 12ers; even the Najdi clergy say the 12er laymen are Muslims, its only the 12er clergy that they make takfeer of.
The reality is, it is not kufr to claim that Imam Ali عليه السلام ورضي الله عنه thought he was an Imam. It maybe a mistake, but it is not kufr, بارك الله فيك.
Unfortunately, by making this post, you have shown your cards, غفر الله لك. My guess is, you probably make takfeer of other Shi'a sects. Heck, I wouldn't be surprised if you made takfeer of the majority of 12ers. However, ولله الحمد, your views are unique to you and can't be attributed to the 12ers.