Is an infidel maidservant, who does not follow any heavenly religion, considered a stranger with whom a Muslim woman is forbidden to be in seclusion?

The correct view is that the 'awrah of a Muslim woman in front of a non-Muslim woman is the same as her 'awrah in front of a Muslim woman. That is, it is permissible for her to see what is above the navel and below the knee. This is because Jewish and idolatrous women in the era of the Prophet (peace be upon him) used to visit the wives of the Prophet and did not veil themselves. It is preferable to replace a non-Muslim maid with a Muslim one, as a non-believer cannot be trusted with a Muslim's home and children, while trying to invite her to Islam with wisdom and good admonition if possible.