Is the 'Umrah valid if I entered into Ihram for it from Jeddah after a flight delay, knowing that my intention was not to perform 'Umrah from the beginning?

Your ihram is valid, and no compensatory sacrifice (dam) is due from you, because you entered into ihram from Jeddah and had not intended to perform Umrah before that point. This is supported by the hadith of Ibn Abbas, which states that the Messenger of Allah, peace and blessings be upon him, appointed the Miqats for their people and for those who pass through them from other than their people, intending Hajj or Umrah. As for those who are within the Miqats, their ihram begins from their own dwellings, and the people of Makkah enter into ihram from Makkah itself. An exception to this is one who intends to enter into ihram for Umrah while in Makkah; such a person must enter into ihram from outside the Haram boundary (al-hill), as Aisha, may Allah be pleased with her, did by the command of the Messenger of Allah, peace and blessings be upon him.