What is the ruling on one who prayed two rak'ahs of 'Isha and made the taslim, then remembered and completed his prayer?

The majority of scholars hold that a single taslima is valid and exits one from prayer. Imam Ahmad, however, was of the opinion that a fard prayer is not complete without two taslimas. The minimum utterance for the taslima is "Assalamu alaykum," and the more complete form adds "wa rahmatullahi." If a worshipper only says "Assalam," they do not exit the prayer. In your case, since you only said "Assalam" and then remembered and stood up to complete the prayer, your prayer is valid. You should have performed the prostration of forgetfulness (sujud (definition) as-sahw) for the addition of "Assalam" and your turning away. If you omitted sujud as-sahw due to forgetfulness or ignorance, your prayer is valid. As for the opening takbir you mentioned, if it was a takbir of transition from the first tashahhud (definition) to standing, then it is valid. If it was a new opening takbir, as if starting the prayer anew, then it is an addition and unnecessary. In this case, you have two reasons for sujud as-sahw, and two prostrations would suffice for both.