As for the question of Imamate, I will mention the following points:
1- Isn’t prayer a pillar of deen? How come the Qur’an does not tell us how many prayers to pray and how many rak’as. Qur’an mentions some very specific details of events or rulings. There is a whole page in Sura al-Baqarah talking about how recommended it is to document a loan. Couldn’t one single verse tell us how to pray? How many units? Do we cross our hands or leave them down? What do we do sujud on? What would a Sunni respond to this. And they accept that prayer is a pillar of faith.
2- What Qur’anic evidence do they have for the legitimacy of the caliphate of the 3 caliphs? Where is one verse that says how is a caliph chosen? What are the requirements? And by the each caliph was chosen differently. Abu Bakr by a very small group at Saqifah. Then Abu Bakr directly appointed Omar. Then Omar appointed a 6-member council. None of these details come from Qur’an or Sunnah.
3- The Qur’an makes it very clear to us who our religious leaders and guardians are. 5:55 tells us who the guardian is, and the one who gave charity while in ruku’ was Imam Ali. That’s sufficient to determine he is our guardian who must be followed.
4- Religion is based on trying us. Not all answers are clearly given to us. We must use our intellect to research our religion. The Prophet (s) made it very clear that after him there shall be 12 Imams/rulers/caliphs/Amirs.
5- If there was a verse clearly mentioning the 12 Imams, the Umayyads and other enemies of Ahlulbayt (a) would have omitted that verse from the Qur’an and this would open the door to distorting the Qur’an. But Allah has promised to protect the Qur’an from any changes, and so one way to protect it is by not revealing a verse that people would definitely omit.