What will happen to H4 and H4 EAD application which is in process, if primary H1B applicant changes job?

Spouse's H-4 approval and EAD remain valid as long as the new H-1B sponsored by Employer B gets approved.

I married a Nigerian in 2015 I found out a couple of weeks he has had a wife in his country every since 2011 is my marriage lega

You can report a fraudulent marriage to ICE. Unfortunately, if he was already married, your marriage is not legal. Family lawyer can advise you...

How can I get a SSN denial letter without a I94 and I20?

Question is unclear. I suggest you seek legal assistance.

Till what time can a L2 via holder work if the primary Visa holder's status has been changed to H1b, picked in lottery?

You can only work as long as he is valid L-1 visa status (and you have an EAD).

Father in law for co sponsor?

A co-sponsor need not be a relative. Once they agree to become one, they are responsible for you not becoming a public charge for 10 years.

Check with the local Clerk of Court in your county or online at its website to find out what proofs you need to obtain a marriage license. You...

Can I work when my OPT STEM extension application is denied due to deficient signature? My 1 year OPT has expired.

There is a deadline to file a motion to re-open. Your employer cannot employ you if your OPT and hence EAD have expired. It is also questionable...

I've I-140 approved in EB1C category. Can I file I-824 by myself with the support of employment letter?

Instructions say that the petitioner must file the I-824. For an I-140, this is your employer. Ask your employer's immigration attorney to assist.

Can I use AVR to return back from canada?

Why risk a potential problem at the border? Suggest you get another F-1 visa in your passport when you return to Canada since you are still enrolled.

The answer is no. He works for a contractor to the US government not for the US government directly.