Is the Umrah valid if the niqab was worn after the sa'i and before shortening the hair, knowing that it is necessary to shorten some hair after the sa'i, and being unaware that the release from ihram comes after it and not before it?

Shaving or shortening the hair in 'Umrah is a ritual and one of its obligations, and one does not exit the state of ihram except by it. As long as you were ignorant that shortening the hair comes before exiting ihram, there is nothing upon you, and your 'Umrah is valid due to your excuse of ignorance.