Is wearing an isdal instead of a khimar forbidden, especially if the husband asks her to wear other clothes on occasions?

There is no specific type or color of clothing that a Muslim woman is obligated to wear. Rather, the important thing is that the clothing covers the entire body—with scholars differing on the face and hands, though the preponderant opinion is the obligation to cover them—and that it be loose and ample, not outlining the body, and thick enough not to reveal the skin. It should also not involve imitating men, disbelievers, or immoral women, and it should not be an adornment that attracts attention. If these specifications are met in the clothing, then there is no objection to wearing it. Otherwise, it is not permissible to wear it, even if the husband commands it; for there is no obedience to a creature in disobedience to the Creator.