Answer

Hello Nitzan.

1. That is exactly what the rule disqualifying them from testimony means. Their testimony is not accepted. In some cases there is no conflict, because their testimony is indeed questionable. But regarding relatives, for example, the Talmud and the halakhic decisors write that they are disqualified from testimony not because there is concern that they are lying, but because it is a scriptural decree. There the question arises even more sharply, and indeed we see that the disqualification overrides the need to arrive at the truth. Apparently there is some reason for which they are disqualified (we do not want them to testify), and that reason is more important to the Torah than missing the truth in that case. I will just note that in monetary law, the judge has discretion to accept any evidence, and not necessarily according to the regular halakhic rules of evidence. Therefore, a person will not lose out because of these disqualifications. Their significance is mainly in testimony relating to halakhic matters, and not to legal disputes between people.
2. Most of these things are beliefs that people have absorbed from an unclear source, and they have no basis whatsoever. On the contrary, someone who believes in them violates Torah prohibitions (“You shall be wholehearted with the Lord your God,” and others). There are a few such beliefs that do indeed appear in the Talmud, but I too am doubtful about them. As a rule, when we are talking about facts, it is impossible to demand that a person believe facts he does not believe in on the authority of the Sages. Even if they demand it, he cannot change what he believes unless you persuade him. Otherwise he can at most say that he believes, but not actually believe. Therefore the question of whether or not he is a heretic has no real meaning.