Is the 'Umrah valid if one performs the ihram from Jeddah after spending the night there, having arrived overland from Riyadh?

You and your wife should have entered into the state of ihram from the miqat of the people of Najd (Qarn al-Manazil/Sail al-Kabir), not from Jeddah, because you passed the miqat without being in ihram. Each of you must offer a sacrifice (dam) to be slaughtered and distributed to the poor of the Haram, and your 'Umrah is valid because entering into ihram from the miqat is an obligation, not a pillar. However, if you came to Jeddah without intending to perform the ritual and then decided to perform 'Umrah, then there is nothing upon you.