Here's what Neville says to explain the appearance.
"It's quite straightforward, really," said Neville modestly. "I'd been in here about a day and a half, and getting really hungry, and wishing I could get something to eat, and that's when the passage to the Hog's Head opened up. I went through it and met Aberforth. He's been providing us with food, because for some reason, that's the one thing the room doesn't do.
Everything he says seems to point towards the fact that the passage appeared after, when Neville was getting hungry.
As for your secondary question, it is simply logic. One would assume that the passage from the seventh floor would be visible from outside the castle, and Harry and Ron would have noticed it, as the Death Eaters would. The passage jumped right from the 7th floor to Ariana's portrait, like a portal, as @Richard said in his coment above. In answer to your comment in response, they did get a portal to Hogsmeade. Ariana's portrait was the optimal place for the exit of such a portal.