السلام عليكم Is what im aware of correct: that we can't testify for individiuals who are non- muslims that they will remain in hellfire forever even if they have heard of islam; and that the only way to remain in the hellfire forever is if they fully understood islam is the truth, and then reject it

Ustadh AbdulAziz al-Haqqan

This is incorrect, as the scholars have mentioned that the message reached him and he was able to come to know it, not that he understood it to be the truth and then rejected it.