Is what the questioner did by returning to his work in Canada after spending two months with his mother and before her death considered an act of disobedience to her, and what is its expiation if it is so?

It appears that you were keen on honoring your mother, and your travel for work is not an act of disobedience. What you feel regarding shortcomings is a sign of sincerity. Continue to honor her after her death by supplicating for her, giving charity on her behalf, maintaining kinship ties, and honoring her friends.