No iconic bunny outfit or Chainsaw Giles? The horror!
The Scoobies might still have managed to get out, though there could well have been more than one death amongst the other partygoers. Xander could still have remembered about the book, but whether that accomplished anything would largely depend on whether Willow could read it.
The Scoobies themselves had managed to regroup and get past their fears, but the place was still sealed up thanks to Gachnar. Willow might have been able to use magic to break down a wall, but her magic was at best only 50/50! Buffy may have had the strength to break down a wooden wall, if she had tried, and she might have broken the symbol of her own accord, once she knew what it was. Or just because it was an occult symbol, given what was going on.
Gachnar, once manifested was, um, less than impressive; the issue was a lack of knowledge. The book had the knowledge they needed, but if they could not read it, it would not have helped.