Is the 'Umrah valid if the Miqat is passed without 'Ihram, and then one returns to it for 'Ihram and to complete the 'Umrah?

If you assume ihram after passing the miqat, then return to the miqat while in ihram, the preponderant view according to the Hanbalis and Malikis is that a compensatory sacrifice (dam) becomes obligatory. This is because a necessary rite was omitted, namely assuming ihram from the miqat. This is the well-known position within the madhhab; merely returning does not waive the compensatory sacrifice. However, if you return to the miqat before assuming ihram and then assume ihram from there, nothing is incumbent upon you.