[Iarhen]

I think it was mostly because of the fact that Rivendell was, in itself, a citadel… Built to protect… Since the citadel itself had protective features, possibly Elrond did not needed to put forth so much power to guard it against evil. Only when the Nazgul approached, he had to raise the river versus the Riders. But other than that, Elrond had a much easier job… Also taking in the fact that Rivendell was a lot smaller than Lorien and that the Misty Mountains protected Rivendell from all the nasties that had their main abode in Mordor, Dol Guldur, Minas Morgul, Isengard, Mirkwood, etc.

And to your second question… I highly doubt that Lorien would eventually fall to Dol Guldur’s attacks. Remember the appendix of ROTK… Besides the valour and courage of the elven people that lived there, the power that dwelt there was far too great for any to overcome, unless Sauron himself came to Lorien to undo the fences that Galadriel wove around her realm. Much harm would be done on the woods of the borders, but Lorien would stand against them…
I think that Dol Guldur would, eventually, run out of forces with each failed assault against Lorien. And even though its quite possible that the Lorien hosts would not get out of the Golden Wood to attack Dol Guldur with Sauron still around, Lorien would have been left in peace till Sauron sent more forces or came there himself… That would be a dread thing to happen… The Dark Lord on the border… killing the elves… And Celeborn and Galadriel trying to find a way out… Thank God it never happened!