Is he who does not invoke Allah's pleasure upon the companions of the Messenger of Allah, peace and blessings be upon him, considered an apostate and disbeliever, and is it permissible to kill him based on the Prophet's saying: "Whoever changes his religion, kill him"?

Merely refraining from invoking Allah's pleasure upon the Companions (may Allah be pleased with them) is not disbelief (kufr), though continuous omission of it is a defect and a sign of rancor and malice. If a specific Muslim says or does something that is legally established as disbelief, he is not to be judged as afir until the conditions are met and the impediments are removed. This ruling is entrusted to the Sharia judiciary or to the righteous, mujtahid scholars. Similarly, the execution of the apostasy punishment is the prerogative of the legitimate ruler (wali al-amr).