Does the discoverability rule of limitations apply to the discoveryability of a cause of action?

Even if the discoverability rule of limitations applies to this case (which I need not decide), it does not call for perfect certainty. It does not require discovery at all: it says something else will do instead. It suffices that "the material facts on which [the cause of action] is based… ought to have been discovered by the plaintiff by the exercise of reasonable diligence…: Central Trust v. Rafuse 1986 CanLII 29 (SCC), [1986] 2 S.C.R. 147, 224. If the plaintiff is told a fact by someone who is likely to know, surely that makes the fact known or discoverable, even if someone else disputes the fact. Very few people who sue have perfect certainty.