Is uttering half of the Shahadah, or stopping at the middle of the Kalimat at-Tawhid, considered apostasy that invalidates the marriage contract, or is that from the whisperings of Satan?

There is no difference between the two scenarios mentioned, and the questioner is suffering from waswasah (obsessive whispering). He should cast aside these thoughts and not allow Satan to gain power over him, for a servant's dislike and aversion to satanic whispers is an evidence of sound belief and strong faith, as the Prophet, peace and blessings be upon him, said: "That is pure faith." The questioner should ignore these whispers and occupy himself with the remembrance of Allah, reciting the Quran, and making du'a. He may also consult specialized doctors.