What is the ruling on feigning prayer behind an imam who seeks aid from the dead, and is this considered major shirk if the motive is shyness from people and not seeking the countenance of Allah?

Firstly: It is not permissible to pray behind someone who seeks help from the dead and supplicates to them instead of Allah.

Secondly: If a person enters the mosque and someone who is not fit to lead the prayer steps forward, and one fears discord, it is permissible to pray individually, conforming with the Imam in bowing, prostrating, standing, and sitting. One would have the intention of praying alone, and their prayer would be valid. Alternatively, one could pray with the Imam with the intention that this prayer will not be counted, and then repeat it. Both options are permissible.