Is it obligatory to repeat the Dhuhr and Asr prayers, if they were combined with delayed timing (jama' ta'kheer), if the worshipper discovers that he prayed them before the entry of the Asr prayer time?

The duration that permits shortening prayers has been clarified previously. If you followed someone whose تقليد (following/emulating) is permissible, then we hope there is no harm upon you, though following the opinion of the majority is safer. The view of Sheikh Ibn 'Uthaymeen — may Allah have mercy on him — is that it is permissible for a traveler to combine prayers absolutely, whether he is on the move or has stopped. He does not consider the intention to combine prayers (نية الجمع) to be a condition for combining them in advance (جمع التقديم). Therefore, your action is correct and sufficient, and you are not required to repeat the Dhuhr prayer.