Not intended to come across as sarcastic (ok, perhaps just a little

)…but what difference does it really make? We will NEVER know. THE END. The oceans of ink spilled over the “Synoptic Problem” is primarily the doings of navel-gazing academics trying to come to grips with this fact…as well as to justify their employment.
Everybody begins from somewhere. One of the first questions asked by those inquisitive about Christianity is why there are 4 Gospels instead of 1. A Good question indeed.
The short answer: the Gospels are at their core the memoirs of specific apostles as handed down orally in the apostolic communities that they themselves founded. Like every re-telling of a story, certain details change depending on: who’s perspective is being narrated, the clarity of recollection and who does the re-telling. Somebody in each community finally decided to write the oral tradition down…thus, the Gospels as we know them.
We live in a literate society today, so we have difficulty in grasping the concept that the Church didn’t have a written New Testament to found themselves around. The second (and third) generation wrote down this oral tradition so the memories could be preserved.: the NT we know today. Wrapping my head around this is what led me to convert to Catholicism.
If your faith is shaken to its very foundations by the possibility that the authors of both Matthew & Luke had before them a copy of Mark and then redacted it according to their own apostolic community’s oral tradition…then that’s more a statement about the fragility of your faith than anything. All the Gospels have as their source the same collective experience of the apostles.