1) Your server would likely die a horrible death if you set up replication on 500 databases, unless they were mostly read-only.

2) Likewise for database mirroring (even asynchronous).

3) I built a custom log-shipping process for a client that managed over 7300 databases on a single server, it was fairly complicated but bullet-proof once I got all the kinks worked out. I even had the backup server go into standby-mode for the restored databases so they could be hit for read-only reporting during the day. This isn't something we can do via forums but I would be happy to engage with you to see if we can modify the system I built to work in your environment to fit your needs.