Is it permissible to travel without a mahram to visit an ailing and elderly grandmother, knowing that there will be trustworthy women and a male relative accompanying the journey? And does adopting the view of Ibn Taymiyyah, which permits travel for an act of obedience without a mahram, remove the sin in this situation?

If the situation is as you mentioned, then there is no blame upon you, Allah willing, for adopting the view of those who permit a woman to travel without a mahram (definition) if accompanied by trustworthy companions for a journey of obedience, to alleviate hardship, and not out of a pursuit of concessions, as some scholars have stipulated.