2. Fred crashes a lot. Its just one of those facts of life. If you want to get your mission back, look in the /data/missions/ folder and look for stuff called Backup.00X (X being 1 to 9). Those are autosaved files that FRED makes every so often. If you change the extention to .fs2, you can get your crashed mission back. Backup.001 will be the most recent.
4. Neither. Send message list uses milliseconds, so your message would run through in a fraction of a second.

To turn it into seconds, just multiply it by 1000.
But if I get what your question is, it is the delay since the last message that played. So "Freespace 2 rules" would happen right away, "Yes we know" would happen 4 after "Freespace 2 rules", and "Ok just checking" would happen 8 after "Yes we know" and 12 after "Freespace 2 rules".