Potato Sack Reunion giveaways (of individual games) are fair game, right?

Nope, they're gifts so there isnt a problem. The only one problem with actual gifts we had was the Ship but those came from Indie Bundle keys that, if you wanted to, you could get for cheap

I'd really like to think the Valve comment "Already own any of the games in this pack? Give your extra copy to a friend." would be carte blanche to give them away here, but yeah, you never know. Hopefully a mod comes in and confirms.

If Steam automatically puts the "extras" in your inventory, I don't see the difference between buying this and any other games that are on sale, personally.

I don't see why you couldn't just because it's in a pack.