All perfect praise be to Allah, The Lord of The Worlds; and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Assuming Ihram for Umrah before having finished Hajj is invalid and doesn`t count, because a pilgrim is busy with performing the rites of Hajj. "
The time for offering Umrah extends throughout the whole year; however, it is impermissible to assume Ihram for Umrah in certain times:
1. Being in a state of Ihram for Umrah.
2. Being in a state of Ihram for Hajj.
3. Assuming Ihram for Umrah before having departed from Mina, because a pilgrim is busy with throwing the pebbles and staying the night in Mina; therefore, he doesn`t have the time to perform the rites of Umrah.
As a result, your Ihram (Asker) for Umrah on behalf of your mother before having offered all rites of Hajj doesn`t count, and the Tawaf that you have performed avails for Tawaf Al-Ifadah; however, you aren`t liable to offer any expiation. And Allah knows best.