Is it permissible for a woman to travel by plane for two hours without a mahram to her husband, who is facing severe trials, knowing that traveling without a mahram is impermissible, and do her husband's circumstances constitute a concession for her?

It is not permissible for a woman to travel without a mahram (definition), and this is the view of the majority of jurists. There is no difference in this regard between a long distance and a short one. An exception to this is if there is an extreme necessity that compels her to travel.