Is it permissible for a menstruating woman who has intended 'Umrah from the miqat to wear the niqab in Makkah until she becomes pure, then remove it for 'Umrah?

As long as you have entered into the state of ihram for Umrah, the ruling of ihram is binding upon you until you exit from it. It is not permissible for you to wear the niqab, as a woman in ihram is forbidden from it.

If you intentionally wear it, knowing the ruling, before exiting from ihram, then you owe a fidyah (expiation), for which you have a choice: fasting for three days, or feeding six poor persons, or sacrificing a sheep.

However, if you wore it out of ignorance or forgetfulness, then no fidyah is due upon you.