1. Yes, you can go for H1B transfer to a new company before October 1st.
2. I am not sure of all the risks associated with that. But one possibility of risk that I can see is, "when B's H1B petition got denied and A revoked their petition". I am not sure if A will come to know about B's petition filing on your behalf to revoke their petition, especially since you haven't started working with them yet.