(1)For example,if USCIS want to check your previous company finacial status,they would send a letter to ask your previous company to provide finacial report of last year.However,since you left,your previous company would not reply this kind of this letter.There is only 1% chance you would meet this kind of this thing.
(2)You can not prevent USCIS not to contact your previous company.You had better not let USCIS know you are laid off now.
(3)Once your I-140 are approved,you are safe and there is no any relation between your previous company and USCIS.You don't need to tell anything to your previous Attorney