No I definitely admit that it is a crack pot theory and would only work if Arthur was somehow manipulating Lancelot as well. What gave me pause was that Lancelot was just hanging out in Arthur’s castle. If he left on bad terms I would be surprised he would not only return to Camelot but also turn up right in the center of the castle. Unless he conveniently heard about Snow and David in Camelot and risked going back just to warn them, which does seem like the sort of thing he would do. My other crazy theory is that this is another fake Lancelot and he really is dead, but I find that very very very unlikely and would be mad if this was the case.
"To leave an enemy without an answer, say these words to him: Aglaria Pidhol garia Ananus Qepta" and blow in his direction; then he will not know which way he is headed and cannot answer you." - Swedish Trollproverb