Does the Twilight Forest have a higher ore density then the normal world?
Technical, it should because of hollow hills, but disregarding them, the ore density appears to be roughly the same from my tests. It's a better place to mine, though, because of the lack of the extra layers above it keeping you from the goodies.
And do all ores spawn in the Twilight Forest?
The biggest issue is that copper spawns between, I believe, 70y and 40y, so you'll end up copper poor if all you do is mine the Twilight Forest.
If you've got GregTech, though, you can fix that via lava cells in an industrial centrifuge.