How could Jesus have been killed before Passover in Mark, if the Last Supper in Mark is the Passover meal, as it says it is in 14:12? They made preparations on the first day, the day of preparations, which is the afternoon of the 14th of Nisan when the lamb is typically sacrificed, to be eaten that evening, the evening being the beginning of the 15th of Nisan. (See Numbers 9:11.) In Mark Jesus was crucified the morning after that meal, which would be the 15th.
But in John Jesus is crucified at the end of the day of preparation (Nisan 14), that is, in the afternoon, precisely when the lamb is supposed to be sacrificed (19:31). John seems to have been following the pauline metaphor (1 Cor. 5:7). So it's exactly as Bart Ehrman says it is in the video--and I mean _exactly_, complete with the likely difference in theological reasoning between the gospel accounts.
You're ignoring the source of the conflict between the gospels, which won't make the problem go away. That problem has occupied the attention of all New Testament scholars, not just William Barclay and Bart Ehrman. Your dispute, then, is with the entirety of the critical-historical profession. For example, Raymond Brown says the gospel accounts can't be easily reconciled.
Are you a biblical inerrantist, by any chance?