Now Rumple by his own admission knows his farsight is imperfect, that said Rumple in the EF is incredibly arrogant as a result of the knowledge he gains from that ability. Pride goeth before a fall as the saying says. If the Wicked Witch is a child of Cora’s and more specifically an elder child then Regina’s tragedy is to have passed through the 9 levels of hell when by rights it should have been the elder sister’s fate.
While I see what you’re saying I also think that you’re neglecting what his foresight did tell him–it told him that Regina would cast the curse. Once you have that information, why bother going sussing out the details? You have what you need (Regina will cast the curse) and while Rumple did look for alternatives (the slippers for instance, or the bean long before that) the fact remains that he believed it was Regina. And if Cora tried (as it appears) to cover up a child from long before Rumple then he wouldn’t even bother going to look for that information because his foresight told him it was Regina.