Without going into rishonim – from my limited exposure to Chabad, nobody discussed mesim when LR was alive and even shortly after. When Rebbe was very sick, the crazy stuff was to declare him moschiach ASAP while he is in this world. It took a little time to re-organize and start a new theory. This _is_ similar to the other religion(s) – when theories change according to events. According to Karl Popper, a real theory is falsifiable – that is it will be declared not true, rather than amended, after some conceivable events. Halochos of false navi are similar.
Maybe, there were previous cases when someone was considered a M aharei misah – previous LR? Also, the nicknames Alter/Mittlele rebbes have a hint of upcoming finality, not sure what exactly happened. If this indeed happened, maybe we can do after-action analysis of those – and how they ended.