Is fasting every Monday sufficient, or is it necessary to fast on Thursdays as well, for someone who cannot fast on Thursdays due to special circumstances?

There is no harm in fasting either Monday or Thursday, without the other. Fasting them both is a Sunnah, not an obligation. Whoever fasts both of them, or one of them, is upon great good. It is not obligatory to combine them; rather, that is recommended due to the authentic narrations reported from the Prophet, peace and blessings be upon him, concerning that.