Oh I think it that's even better, Sauron & co have continued to fortify the inner fortress at least, (maybe let it slide a bit with the rest of the city) and then Luthien comes in and basically blows up the whole fortress, opens up the pits, rescues the prisoners and then around them everything crashes down, the island collapses, the river floods the pits and the lower city and flushes out all the baddies that haven't yet been crushed. And then we only need a nice way for them to get to safety.
Also wow, Beren and Luthien always seemed so far away in Silm Film terms, but now it's like... soon-ish..