AC-21 Portability ,New employer is not ready to give letter :Urgent

Here is my Situation

They started my GC process.

PERM DATE : Feb 6, 2009

PERM approved in DEC 17, 2009

I-140 Approved in Jan 22, 2010.

I-485/EAD/AOS applied Feb 16,2012

EAD/AOS approved : April 1,2012

EAD renewed now till APril 2015

I-485 Still pending

I was working for Company A and they applied GC for me. I worked with them till Dec 16,2012.
I joined A full time position in company B on Dec 17,2012 ( that is I joined Company B more than 180 days after I-485 applied)
but did not apply AC-21.

Now I have a full time offer from another company C which I would like to join but they are saying they will not provide any customize letter for filing Ac-21 if I want to use.

They are saying you can Use
1. Offer letter
2. Job Posting details copy which contains all the job description: as on their web site
3. They can provide employee verification Letter:confused:

Do these information is sufficient for applying AC-21? If Job posting can be used to verify Job description?

Is their any risk joining them?