1) To what extent must a child go in order to not transgress the obligation to fear their parents?

The Talmud (Kidushin 31a) and Shulchan Aruch (Yoreh Deah Siman 240:3) illustrate this as follows:

"If a son is dressed in fine clothing and is sitting at the head of the table presiding over a congregation and his mother or father walk up to him and tear his clothing, hit him on the head and spit in his face, he should not berate or insult them; rather he should remain silent and fear the King of Kings [Hashem] who commanded him to do so."

There is a lot of discussion in the Poskim about the aforementioned passage in the Talmud, and many  Poskim rule that this only applies to a parent that is suffering from Dementia, Alzheimer's or a similar condition where they are not in control of their faculties, but does not apply to normal, healthy parents who should know and act  better. (See Tosefos to Kidushin ibid. Dibur Hamaschil U'Bas imo. See also Midrash Devarim Perek 1:15)

Other Poskim rule that this is the case only "after the fact", but if it did not happen yet and the child can prevent it from happening, he may do so. (See Yam Shel Shlomo Kidushin 31a, Siman 64, for a lengthy treatment of this topic)

2) If a parent is a wicked person and/or abusive to a child, most Poskim indeed rule that the child need not passively suffer and take the abuse, and may defend themselves and rebuke the parent for their inexcusable actions or do anything else that needs to be done to save themselves from an abusive situation. Nobody should suffer in silence. Nobody.

This topic is too lengthy and detailed for this forum to give it a proper treatment.

If Chas V'Shalom someone is in such a situation (or in any abusive situation, for that matter), a Rav (or competent Frum therapist) should be consulted for the best halachically sanctioned course of action. There is no prohibition of Lashon Hara in relating to the Rav (or therapist) all the pertinent details necessary for him or her  to fully understand the situation. (See Chofetz Chaim Hilchos Isurei Lashon Hara Klal 10:14 and in the footnote)