Was leaving arguments and disputes the right thing to do, or am I incurring a sin? And was Al-Siddiq right in his opinion regarding the classification of hearts and hypocrisy? And when does the matter of the heart even become manifest? And what should I do?

The division of hearts into four categories is not Ibn al-Qayyim's own statement, but rather he attributed it to the Companions. Thus, the objector is criticizing the Companions. The objector's assumption that all hypocrisy is hidden is incorrect, as some hypocrisy is manifest and has signs, like the case of the Zanādiqah (heretics) who outwardly profess Islam while concealing disbelief. The Prophet, peace and blessings be upon him, knew some hypocrites, such as Ibn Salul and others. As for major hypocrisy, it is often hidden, and in this case, a Muslim is treated based on what is apparent from them, and their reckoning remains with Allah. However, if the hypocrite reveals their hypocrisy and heresy, they are treated as they deserve.