Is it permissible for a woman to go to work on her day off at the request of her boss, given the presence of a few men or a male colleague, with the possibility or not of another woman being present, and is this considered khalwa (unlawful seclusion) and what is its ruling in each case?

The ruling regarding a woman going to work on a holiday does not differ from other days; it is permissible for her to do so if she adheres to the Sharia guidelines. It is not permissible for her to be alone with a man. As for her presence with two or three men, that has its own detailed ruling.