The Live Wall

I was wondering about what was actually live and what was not during the wall shows. For example i have read that the song "Is there anybody out there" is not live and the album version is played with gilmour playing the live guitar at the end. Is the piano during "Nobody home" live or is that a studio track that waters sings to? Also I keep reading that empty spaces was not played live yet on the "IS there anybody out there" disc it is listed, and during the first LA Show this is the song during which the curtain caught fire. Why is the song constantly missed?
So to sum it up for those that got lost in my ramble: which parts of the wall are actually played by the musicians and not a backign track? Alas one more: are there any overdubs on the set "Is there anybody out there?"