To what extent is the story of the Companion who added "Much pure, blessed praise" after rising from bowing (ruku') valid as evidence for the permissibility of "bid'ah hasanah" (good innovation), and does Ibn Hajar's description of Ibn Taymiyyah as "a slave who leads others to misguidance" support this view?

There can be no "good innovation" (bid'ah hasanah) because of the Prophet's (peace and blessings be upon him) saying: "Every innovation is misguidance, and every misguidance is in the Fire." And the praise after rising from bowing is a remembrance firmly established from the Prophet (peace and blessings be upon him), and the Companion (Sahabi) used one of the forms of praise, which is not an indication of inventing new acts of worship. And the action of a Companion is not a proof unless it is approved by the Prophet (peace and blessings be upon him), and this is not possible after his death. The noble hadith "Every innovation is misguidance" is general, and the incident of the Companion is a specific event that does not indicate generality. And "good" cannot be known without revelation, because it would lead to disorder and disagreement. What the Prophet (peace and blessings be upon him) approved of the Companion's action becomes a good sunnah, but after his death and the cessation of revelation, it is impossible to know whether newly invented acts of worship are correct. Therefore, one must limit oneself to what has been transmitted in the Sharia. And Ibn Hajar's نقل (transmission) from Ibn Taymiyyah, whether in objection or agreement, is a matter for scholars in their discussions to reach the truth. As for the bad statement mentioned in the question, it is false, considering Ibn Hajar's etiquette and knowledge.