As stated, Milk-Bosatsu is probably fake. The story straight up doesn't add up. His source saw this character Last November/December in a way that they couldn't tell who it was. At the very least, that suggests a textureless model. Back then, Sakurai only stated that Characters 1, 2 and 3 were being developed. So the only way Milk's claim makes any sense is if the supposed character is Kasumi from Persona 5 and She was indeed planned as an alt for Joker before it ultimately got scrapped.
Smash has never ended on a Bang and even then, Reimu wouldn't be the end of DLC, so the need for the bang is completely unnecessary. I've also said it before in this thread, the announcement of more DLC before the Pass has been fully revealed is weird to me on almost every level. To me, it suggests a divisive character is coming, moreso than Terry was. That way said character isn't viewed as taking up the last slot before announcing more DLC like a more traditional fighting game would handle "season pass" announcements.