Is it permissible to perform the absentee funeral prayer (Salat al-Gha'ib) for deceased Muslims after the Isha' prayer every day, and does this prayer benefit them?

What you intend to do, praying the funeral prayer for an absent deceased every night, is an innovation (bid‘ah) because it has not been transmitted from the righteous predecessors (Salaf al-Salih). Shaykh al-Islam Ibn Taymiyyah said: "Nor is the funeral prayer performed every day for an absent person, because it has not been transmitted." What some people do, performing the funeral prayer every night for all Muslims who died on that day, is undoubtedly an innovation. The Prophet, peace and blessings be upon him, said: "Whoever introduces into this matter of ours that which is not from it will have it rejected."