There are three types of knowledge:
* The genuine unseen. This is what Allah knows only.
* Knowledge that is restricted but not completely unknown. For instance, when the Prophet (S) went on the mi'raj, he saw things that are generally inaccessible to living human beings. Most human beings also do not directly see or hear the angel Jibra'il. Etc.
* Knowledge that is conventionally accessible. For instance, a book in a library - it can be read by anyone who can read the language and access the book.
This question seems to hinge on the second and third types of knowledge.
Shi'is have held a range of views regarding the knowledge possessed by the prophets and Imams. It is not necessary to adhere to a specific view on this in order to be a Shi'i, although a general bare minimum view for the Ithna Ashari tradition is that the prophets and Imams do not make mistakes and always offer correct guidance, especially although not only on religious matters.
However, my understanding of Ithna Ashari Shi'i texts is that the Prophet Muhammad (S), the twelve Imams (A), and Fatima al-Zahra (A) possessed or had the ability to access all the knowledge possessed by the previous prophets, and also had a comprehensive knowledge of the world and worldly matters, religious law, the true meanings of the Qur'an, metaphysical matters (as much as is possible), and knowledge of the past and future. Therefore, they would not make mistakes or misguide people or lose people's confidence by saying something wrong. This persists in their otherworldly condition, although perhaps, since Allah is infinite, their knowledge of metaphysical realities can continue to expand.
This view is based on narrations in books such as al-Kafi with respect to the discussions of their knowledge in this life as well as things such as the world of pre-creation. Of course, in the afterlife, all of our knowledge will expand because we will see new realities and will become aware of the truth of some things that were hidden from us or which we were confused about in this world.
Here is a short treatment of the subject with some narrations: https://www.al-islam.org/imamate-and-leadership-sayyid-mujtaba-musavi-lari/lesson-21-sources-imams-knowledge
However, if someone is genuinely committed to following the Imams, I think it is good for them to think this over personally rather than to just to take someone's word for it; it is good to spend time reading and pondering over what is narrated, asking Allah for guidance, and coming to their own understanding. We live in an age where these materials are readily available to us.