What is the ruling regarding the intrusive thoughts I experience concerning polytheism in words I utter inadvertently or unintentionally, or actions I perform without the intention of associating partners with Allah? Am I sinful for that? And how can I get rid of these intrusive thoughts?

You are afflicted with whisperings (waswasah) concerning creed and purity. The cure for this is to completely turn away from giving in to the devil. Be certain that merely making a mistake in a letter does not lead to disbelief (kufr), because it is pardoned. Do not obey the devil by abandoning remembrance (dhikr), for remembrance is a fortress against the devil. As for swearing by other than Allah, it is a minor polytheism (shirk asghar), and its cure is to say "Lā ilāha illallāh" (There is no god but Allah). And be certain that your repentance is accepted, Allah willing.