Have I fallen into polytheism (shirk) or disbelief (kufr) because of my resorting to a شيخ (sheikh) who claims to treat with Muslim jinn, and what are the signs of accepted repentance, and is it possible that he has harmed me spiritually after I refused some of his requests?

Seeking assistance from jinn is absolutely impermissible, and one must repent to Allah for it. Proper repentance involves regret and abstaining from committing the sin. If repentance fulfills its conditions, it is accepted. There is nothing further required of you beyond what you have already done, and do not fear any harm, for Allah is the Benefactor and the Harmer, and upon Him alone is reliance. Increase in ruqyah for yourself with the established adhkar and supplications.