1. Is normal ssh access allowed ?
2. How exactly does LISH work ? (ssh keys depend on ssh, if the network is down thats not going to get you far -- wrapper around xm console ? dom0 based vs domU ?)
3. Can I order 2 nodes and have them in the same DC but on different hosts ? (utilizing the private 'lan')
1) Yes, you can run whatever services you like on your Linode, including SSH.
2) Using Lish, you make an SSH connection to the actual server that hosts your Linode. It works regardless of the status of your Linode's network. It's basically like walking up to the physical terminal of a box.
3) Yes, and I don't believe there's ever a bandwidth charge for transfer between Linodes at the same DC.