1. There was a poster here who said he had a Wii and all the instruments then bought a Wii U and would try out all the instruments to see if they where completely compatible. I don't know if he got back to us, but I'm guessing no. And if that's true then he's a jerk for teasing us about it. But from what I hear, I don't really see any reason why all the instruments wouldn't work. But until I know for sure, I'm not going to verify that and instead just say, "they should all work."
2. Yes. Wii U basically emulates the Wii so the game and internet connection think you are playing a Wii and not a Wii U. Everything should be compatible DLC wise.
The Wii version gets practically every official song released by Harmonix. By official I mean not Rock Band Network. The few exceptions are things like the complete version of RUSH's 2112 and some others. It's not because they didn't want to it's because the songs where too big to be compatible on the Wii. As for what is missing, I don't think you can find an official list but it's practically the whole RBN catalog. Not all of them since a few where actually ported before Harmonix deemed it financially unviable and pulled the plug on RBN for the Wii.
As someone already mentioned, the Wii didn't get Rock Band Blitz so Harmonix is instead slowly releasing the RBB songs as separate DLC for all systems including the Wii.
Also the last thing to point out is that Wii/Wii U version of RB3 cannot import;
Rock Band 1
I own a Wii and saddened that I can't get RB1 and Lego Rb. There are some good song there I'd like to play. Ac/DC is okay, but nothing worth crying over. Still hoping for the original versions of AC/DC anyways, hopefully with Pro Upgrades.