H1B transfer - Start date after i797a? - Start PERM process before i797a?

Hello All,

I have a few questions about the h1 transfer process. I have currently accepted an offer from company B and in the process of transferring the H1. The lawyers have received the i797 c intimating that the application has been received. Now

1. Is it legal for me to start working with just the i797 c ? The document states clearly that it does not grant me any immigration status or benefit 
2. I am not sure if the company has filed for premium processing or not. If they haven't i assume it will take 3 months from now to get the approved notice. Can i work on the i797c till then?
3. Do i need to have a fully approved i797 a to actually start working for the new employer? 
4. Finally, can the firm start the PERM process without the fully approved i797 a? 

Thanks in advance your time & help