Learn English – “I’ll let you know” variations

I agree with @MaulikV's answer that "second-to-last" is a natural way to describe it; "next-to-last" would also be perfectly normal and possibly even more common.

Other ways to describe it would be to say "last-but-one" or something like "6th out of 7."

To answer your "broadened usage" question, yes, you can go from best to next-best or second-best; in these cases, the preposition to is not required (indeed, it must be omitted).

"Guess what?" Is usually used when you want to start a conversation on a new and interesting topic. Usually, this will be something that you think will surprise the listener or is something exciting that the listener will be happy to hear.

Guess what? I finally got that job I interviewed for!
Guess what? Jessica and Tim got married! Can you believe it?

"You know what?" Is generally used during conversation when you want to change the direction or topic of discussion. It is usually said abruptly and followed by a statement with a tone of finality.

You know what? All this sad talk is making me want to cry. Let's talk about something else.
You know what? I just realized that Jane was lying to me the whole time. She can be really selfish sometimes.