Does what I did — going to a sorcerer and carrying out his orders unwillingly, under duress — count as Shirk or one of the nullifiers of Islam, thereby invalidating my deeds and requiring repentance, knowing that my heart was not at ease with these actions and I was forced into them?

The aforementioned ruqyah (exorcism/recitation for healing) is an innovated practice (bid'ah), not legislated (unlawful). However, it does not reach the level of magic or disbelief (kufr), nor does it nullify good deeds entirely, except for one who dies upon disbelief. For one who apostatizes and then returns to Islam, or repents from sins, their previous good deeds are restored to them. Obedience to parents is not obligatory in seeking treatment from individuals like this person.