Is it permissible for a girl to wear the khimar and take it off sometimes before settling on it, or is that considered hypocrisy?

There is no disagreement among scholars regarding the obligation for a mature Muslim woman to cover her head and the rest of her body, except for her face and hands, from all non-mahram men. It is not permissible for her to expose any part of her 'awrah. The preponderant opinion is that it is also obligatory to cover the face and hands. Based on this, it is not permissible for a Muslim woman to temporarily remove her hijab in the presence of non-mahram men.