Is the 'Umrah I performed valid, or do I owe an expiation, and what is it if due?

Whoever passes the Miqat without consecrating himself (i.e., entering into a state of Ihram) must return to it to consecrate himself from there. If he consecrates himself from Jeddah, a compensatory sacrifice (dam jubran) becomes obligatory upon him for having bypassed the Miqat. You were correct in returning to the Miqat and consecrating yourself from there. Your doffing of the Ihram garments does not necessitate a compensatory sacrifice because you had not yet entered into a state of Ihram to begin with. Therefore, your action is correct, and no expiation is required of you.