H1B Transfer edge scenarios

Currently in H1B. Moving from company A to B. Company B is doing Premium. I’m planning to inform my company A as soon I get my receipt number , that I’m resigning due their notice Period of 15 to 30 days. Even company B wants to hire ASAP after h1 approved.

  1. Is it ok inform Company A when I get my receipt number ? Any advice
  2. If company A ends by employments ASAP before my start date in Company B. Can I make use of 60 days grace period ?
  3. Once I inform company A after my receipt can they do any damage to my h1b (typical IT firm strategy ) to hold back the candidate and etc.

Plz advise , thanks in advance !