Proof for what?
If a company is giving layoffs, normally there are termination notices of some sort even if it is one sentence. So it is not clear what proof you are refering to?
Withdrawal of H1 is to be done by the employer at a later date depending on how busy their HR department is.
Another issue is about filing for H4 after filing for I-485, can this be done? I am not sure about it, a lawyer will be able to clarify.