Is mocking and bullying others—whether they are known or not, and in their absence—considered forbidden (haram), and can its effect rebound negatively on the perpetrator?

What you are doing by mocking others is forbidden by Islamic law. Allah Almighty has prohibited mockery in His saying: "O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them; nor let women ridicule [other] women; perhaps they may be better than them." If the mockery occurs in the absence of the person being mocked, it includes backbiting, which is among the major sins. You must repent to Allah for this sin and refrain from mocking anyone, whether in their presence or absence. Regarding expiation for backbiting, you can refer to Fatwa no. 122563. Furthermore, it is not permissible to joke with your fiancée before the marriage contract because she is a stranger [non-mahram], and it is not permissible to speak with her except to the extent of necessity.