Please address these "queries" to the employer's immigration attorney. Part of their responsibility is to talk to you and advise you.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.
1. None other than an approved PERM, the employer's proof of it being able to pay the proffered wage mandated by DOL and your proof of meeting the job's educational and/or on the job experience requirements. The ability to pay requirement on the employer "attaches" on the year your "priority date" is established and continues thereafter until the very date in which you are admitted to permanent residence.
2. Yes. No "implications". The PERM and I-140 are job offers for the future. There is no requirement that you currently work for the petitioning employer.
3. Yes, Labor is an animal of its own, with no relation and independent of the H-1B. (For ":Labor" to continue to be valid, however, employer is obligated to file an I-140 petition no later than within 6 months of the Labor certification's approval.)
Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
Direct all questions to Employer B's attorney, who is best placed to advise you as they have your documented immigration history. If completely non-responsive, notify the employer.
Co. B should be capable of paying the offered salary when filing I-140 and should intend to employ you for an indefinite term in the future (when you receive your green card).
You are not required to be employed by Co. B in the interim.
The GC process through employer sponsorship can continue even if you are not present in the U.S. or employed elsewhere, but you cannot remain legally in the U.S. solely on the basis of PERM/I-140.