If a disbeliever or apostate becomes Muslim but persists in committing a sin from which he has not repented, his becoming Muslim is valid according to scholarly consensus. But will he be forgiven for his sin from which he has not repented by virtue of becoming Muslim? Concerning that there are two scholarly views. See a detailed discussion of that in the long answer.

The words that were attributed to ‘Ali (may Allah be pleased with him) – Did you come to know your Lord through Muhammad…? – are words that are to be found in the books of the Shi‘ah as part of a long story. The evidence that this is a lie is obvious in this report, as they use it to impugn Abu Bakr and ‘Umar (may Allah be pleased with them) and cast aspersions on their knowledge, and they narrate it with an isnaad that is not free of narrators who were accused of lying and narrators who are unknown.

It is not permissible to ask fortunetellers questions, because the Prophet (blessings and peace of Allah be upon him) said: Whoever goes to a fortuneteller and asks him about anything, no prayer will be accepted from him for forty days.” Narrated by Muslim (2230). This applies to the one who asked him a question without believing him. However, if he also believes that, then the matter is more serious, as the Prophet (blessings and peace of Allah be upon him) said: “Whoever has intercourse with a menstruating woman, or with a woman in her back passage, or goes to a fortuneteller and believes what he says, has disbelieved in what Allah sent down to Muhammad (blessings and peace of Allah be upon him).” Reading the words of fortune tellers and soothsayers is haraam (forbidden); it is like asking them questions. If you deliberately read their words, then repent to Allah and seek His forgiveness, and do not do such a thing ever again.