1. Depends on how else you're protecting the catalog. If you have an Appliance Master, don't keep your only catalog backup copy on the local MSDP.
2. Yes, it's actually required to be on MSDP if you are planning on replicating the backup via AIR. If you're making a copy to tape it's optional.
3. If it's on local MSDP on the Master, you would have to recover the MSDP catalog first before you can get access to any of the images on the MSDP. Hopefully you stored the MSDP catalog backup on a different server, or at least on Basic or Adv. disk instead of MSDP ? Once NetBackup knows about the MSDP images you ought to be able to recover normally.
4. If you can't make extra copies of the catalog offsite for some reason, then I would plan on keeping several on Basic or Advanced disk - no reason you can't keep some on MSDP as well but don't have that be your only available copy.
5. No other environmental impact, you can change how your catalog backups are setup at any time.
6. In a perfect world you'd be backing up the catalog to the MSDP on a separate local Media Server, then replicating that catalog backup to your DR site via AIR. If you had a need for longer term retention for some reason (once/month just-in-case copy?), cut tape off of either the Media Server or DR server copies as well and then send that to your vault.