Is there repentance for someone who curses another Muslim, praying for them to enter Hell? And is it possible for this curser to enter Paradise, despite the possibility that their prayer might be answered, causing them to enter Hell?

It is not permissible to supplicate against a Muslim unless he is deserving of it, as the Prophet, peace and blessings be upon him, said: "Whoever says to a Muslim: 'O enemy of Allah,' or supplicates against him, and he is not so, then it will recoil upon him." So, whoever supplicates against a Muslim who does not deserve it, must repent, seek forgiveness, and ask for pardon from the one he supplicated against. Allah accepts his repentance, and this does not mean eternal damnation in Hellfire.