mailbfr thinks the path I have given is not correct?

Most likely your path contains spaces. This is generally not advisable. If you cannot take the spaces out of your path for some reason, you will need to "escape" them.

I restored my mail server. Now I see messages/mailboxes that were deleted?

mailbfr does an incremental backup by default. So unless you change the backup location regularly you may end up with old messages. This is by design otherwise you couldn’t go back to an old messsage a user might request. If you do not wish this behavior, use the "full" or "rotate" option.

Where is the crontab file located for mailbfr?

mailbfr uses the root crontab. You can either edit it directly or use mailbfr -i to add/remove the cron job.