Is there any legitimate religious justification for feeling embarrassed about transferring a Christian maid's salary to her family in December, for fear of it being used to celebrate their holidays, and would a Muslim apostatize if he were to do so?

It is permissible to assist the maid with her transfer at any time, and your suspicion, stemming from waswasah (whisperings of Satan), does not prohibit this. These doubts are remote possibilities that the Shari'ah did not command to block, because doing so would lead to the prohibition of many permissible things. Al-Qarafi mentioned that sadd al-dhara'i' (blocking the means to evil) is not exclusive to the Maliki school of thought, and that dhara'i' (means/pretexts) are of three types: those agreed upon to be blocked, those agreed upon not to be blocked, and those over which there is disagreement.