As to dating the Gospels then we have shockingly little to be certain of. What appear to be terminus ad quem are rather tissue thin arguments. It is quite possible the Gospels as we have them were, shall we say, popularized very late easily the mid 2nd century. What we do see in all the church writers of the first few centuries is a huge diversity of understanding of a narrative. e.g When this Christ died, if he died, if he had a prehuman form, did he do miracles, his nativity, on and on, the most basic stuff a Sunday school student could recite was not a consensus for hundreds of years. Surely this bears on this question, church leaders of the first centuries CE either did not know about these stories or they knew others or maybe they knew many stories but preferred some not now preserved, it's a deeply complex issue. Indisputably however the 4 Gospels in our modern canon were not the foundational documents of the earliest Christians. . I fear we may never know much more till another cache of writings is unearthed.