I know on the box it says 1-2, 3-4 with an extra core set. Does anyone in the know, or who had access to the demo, know if it will be playable at 5 players?

I'm hoping to use this as the go to "play when one of the players can't make it" on PnP RPG night. But, we have 6 players, so if one is missing that makes 5.

Any thoughts?

It really depends on how well the game as a whole scales. If the designer didn't intend on it going to 5, it may skew balance in some ways. I'm aware that some things scale, like clues needed, clues per location, and what seems to be boss monster health, but other things may shift and become too easy or difficult with the number of bodies doing things. If that doesn't matter to you, I can imagine it's at least doable, just not recommended.

Based on my experience playing some LotR scenarios with five, I'm gonna say you can certainly do it, just be prepared for some wackiness. Looking at AH, it looks right now like it scales more smoothly than LotR, so going from four to five might not break it. Since the main method of scaling (that we have seen) is number of clues you need, make sure you have at least a couple of strong clue gatherers.

You can do what you want and I suspect this game will scale better than LotR but keep in mind you're probably going to have more downtime and possibly other minor issues. I expect that you can do it, but it likely won't be an optimal experience.

Be ready to house rule some stuff at times. But yea, same as others. You should be able to make it work