Sorry for the late reply.
1. Its ok even if you havent send the formal letter. The point here is that you did make them aware of loss.
2. I guess automatic reply is also good.
3. you can write a new letter for sure. thats no harm. Just mentioned the context of earlier letters. So that they know this is a follow-up + claim + complaint letter.
4. Definately you can always mention all the disappointment and pain you have to go through this.
However please make sure that you dont use the abusive language which can be used against you. Dont use language like "XYZ airlines has worst service in the world" rather say like "I express total dissatifaction with service offered to me by XYZ airlines."

Do see a consumer court / research for the Cosumer protection organizations where you can file case against airlines.

Pavan