Does the prayer, according to Ibn Taymiyyah, become void if it lacks khushu' (devotion/humility), and does that imply the invalidity of the prayer of a congregant behind an imam who does not possess khushu'? And is it permissible to adopt the view of Ibn Uthaymeen on this issue?

As long as you do not know that the Imam lacked devotion in his prayer, your prayer is valid and you are not required to repeat it. Even if you did know that, your prayer is still valid, for the prayer of the congregant is not invalidated by what the Imam does, which the congregant perceives as invalidating the prayer. This is the opinion of the majority of the Salaf.