Does merely thinking about the temptations of disbelief and hellfire, and fearing them, along with performing righteous deeds, adhering to God's law, and uttering the Two Testimonies, provide salvation from hellfire, or is it considered disbelief despite attempts to resist it?

You are not an unbeliever. What you feel is proof of the soundness of your belief, your veneration of God, and your fear of Him. This is a [waswasa] (whisper/insinuation) that God has forgiven. For God has pardoned my Ummah for what their souls whisper or تحدث themselves with, as long as they do not act upon it or speak of it. Your dislike of these satanic thoughts is a sign of thealthy faith. The Companions asked the Prophet, peace and blessings be upon him, about the existence of such thoughts, and he said: "That is pure faith." Waswasa differs from doubt; a person has no choice in it, and their dislike of it indicates faith. As for doubt, it is hesitation between two things. You must turn away from these whisperings and occupy yourself with the remembrance of God, supplication, reciting the Quran, and everything that benefits you.