Is it permissible in the Hanbali Madhhab for a muslimah to take off her hijab in front of her non-muslim father-in-law? For example in the case when she is married to someone who converted/reverted to Islam. And if it’s permissible, is it also advised to do so?
Yes it is permissible to do so as stated by Imam ibn Qudamah and others. That is the legal ruling. As for it being advisable, then this goes back to the nature of her father in law. If he can be trusted, then there is no harm in it.
Nonetheless, this refers to revealing that which is still reasonable and modest, such as the hair and the arms and so on. It does not mean that one can be totally loose with their dress code.
And success is from Allah