Does screaming at one's parents to force them to fulfill a request, and not speaking to them to achieve that, constitute disobedience? And what is the way for sincere repentance from this act?

Raising one's voice at parents and shunning them by not speaking to them are acts of disobedience (uquq) that are forbidden and considered among the gravest of major sins. It is obligatory to show kindness and benevolence to parents absolutely, even if they treat one badly, and even if they are polytheists. Bad temper is not an excuse for mistreating them. It is obligatory to repent from such disobedience and to promptly begin showing kindness to them.