Is it true that if I don't stay paak and stay najis like in junub, where I'm in need of making ghusl e janabat, for a long time purposely, then the bad jins might hurt me or slap me? Can this happen?

No this all myths but it’s Makrooh to stay remain for long hours in the state of Janabat and if Ghusl is not possible soon at least one should do Wudhu before eating and drinking.