Is it permissible to sit in a room isolated from the rest of the house with a non-Muslim maid present, when no other place is available?

If you are secure from temptation (fitna), there is no منع (impediment) to your remaining in the room with the door leading to the other room closed. This would not be considered unlawful khalwa (definition) (private seclusion). However, if you fear falling into sin, do not stay in the house alone with the maidservant. In such a case, either leave the house until your family returns, or seek someone to stay with you in the house who will shield you from temptation.