Another couple of suggestions as to why Denethor should have known where Imladris and the House of Elrond was : -
After the Fellowship left Lorien, Aragorn stated that light boats used to "come out of Wilderland" to Osgiliath a few years previously, certainly during Denethor's lifetime. And the West - East trade route across Eriador and Wilderland from the Ered Luin to Dale, which included Rivendell, would certainly have crossed the North - South route down Anduin and surely some travellers would have used both routes.
When Aragorn served Ecthelion in the guise of Thorongil, surely in conversation with him he might have mentioned Rivendell ?
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