Hello, Rahul. Christian Spiritist here, try and answer the best I can.
1) first and foremost, because without'em, this planet could not support life at all. thei were necessary to ecological balance, to make the soil fertile, in the end, to keep this wonderful machine, this amazing, complex coletive being God Created and we call Earth working and well. Also
- to serve ans a fundamental key on evolution and in the development of the human race as we know today.
- to help men in their chores.
2) hm, tough one. I still don't have an personal opinion about this, need to learn more. What I tend to believe is that animals have a... Vital Principle, a Spark from the Creator. It's not a complete soul, with all the free will it receives, but is a start. Also they don't act in pure instinct, otherwise would not be so many relates of animals in the afterlife. Probably this Vital Principle evolves, like everything, and, with the permission of God, can became a human soul, starting simple and ignorant. But, once on this stage, it will not go back to an animal body.
3) and 4) guess I answered already. Can't say much about that, because I don't know
5) It is not forbidden, but adviced against, specially in the days one will work on the spiritist house, transmiting Jesus Words to the incarnated and desincarnated brothers. the reason is phisiologic. Meat is very hard to be digested and can cause... disconfort, may turn the body more... heavy, more earth-ly, making more difficult the contact with the workers from the other plane. But forbidden, forbidden,... nope, it isn't.