Is Ibn Umar considered an innovator for visiting the grave of the Prophet (PBUH) and of Abu Bakr and Umar upon his return from travel, based on an action not narrated from other companions, and which Sheikh al-Islam regarded as a pure innovation?

There is no problem with the aforementioned statement, nor is there any room for doubt in it. Indeed, Ibn 'Umar used to greet [the Prophet] and then depart without standing to supplicate at the grave. Through this action of his, Shaykh al-Islam and others inferred the impermissibility of standing to supplicate at the grave of the Prophet, peace and blessings be upon him.