Does my uttering the phrase "I am no longer a Muslim" - with the intention of repelling obsessive thoughts - affect my marriage, and will my searching for and writing down words like "disbelief" and "divorce" with the intention of inquiring about them be held against me?

Your turning away from the whispers of swearing by disavowing Islam is a forbidden matter, but it is not considered disbelief and does not affect the validity of your marriage. The majority of jurists hold that there is no expiation for it unless disbelief is concealed in the heart. Some consider it an oath requiring expiation, but the first opinion is sounder. Your marriage is valid, and nothing is required of you by writing or uttering these words with the intention of inquiring. Divorce does not occur by your husband's saying it as long as he did not intend it, nor does it occur by your searching for it or writing it, as all of this is from the whispers that must be repelled.