What should I do after filing cos application?

Your situation is a bit complex. There are ramifications if you withdraw your COS application. Talk to an immigration attorney in private.

Visa stamping clarifications

Talk to the attorney who filed your H-1B petition.

You cannot have two visas/status at the same time.

Do I need an EAD if I have SSN?

As my colleagues have indicated, yes, you need EAD in order to be able to work.

The best answer is that you have to try it so you know it. This decision is up to the immigration officer at the consulate.

What if my I-94 is not found and I'm filling out the i-130 and i-485 forms?

Your admission number must be shown on the stamp. Just send them a copy of your stamped card which would suffice.

Can you remarry once you've got your divorce papers? Annulment pending in other country?

You may ask a family attorney for assurance, but all prior decrees must be final before remarrying.

Can a green card holder stay outside the US for more then 18 months straight and still go back to the US? Without losing his GC?

Yes. It is possible. Look for a good immigration attorney.

What kind of lawyer would I be looking for?

Congrats. You can find Michigan lawyer from Avvo. com.

Employment Authorization Card not delivered by USPS

There can be an issue from USCIS. Contact USCIS and ask them.