Is it permissible to perform Umrah on behalf of the deceased first with the intention of Taqseer (cutting a short amount of hair), then perform Umrah on one's own behalf with the intention of Halaq (shaving the head completely), knowing that I have not performed Umrah for myself before?

A person is not permitted to perform Umrah on behalf of another unless he has already performed it for himself. Umrah in this regard is like Hajj. Therefore, whoever is obligated to perform the Umrah of Islam, or a compensatory Umrah, or a votive Umrah, is not allowed to perform Umrah on behalf of another.

As for returning to a sin after repenting from it, repentance is obligatory. Whoever falls into sin again must renew their repentance whenever they return to it. One should not despair of Allah’s mercy, for Satan wants a person to despair of repentance, and the believer, whenever he sins, repents.