Urgent, Help needed for H1B and AC21 issue after losing job pre 180 days limit

Hi All,
I have a urgent situation that needs your helps, suggestions, expertise and experience: I submitted my I-140 in Feb, 2009 and it has been approved in 2009. I then submitted my I-485 in Feb 2012 and it is still pending now. I have obtained EAD/AP combo card in March 2012. I then lost my job in May 2012, with my I-485 pending around 90 days at the time of layoff. The attorney withdrew my H1B at the time of layoff but kept I-140 intact. Now, at the end of Nov 2012, my previous employer would like to offer me my previous job back: same job title, same duty, same location and salary, same everything, after I have been without a job since the end of May 2012. My I-485 as of now has been pending for 270 days. And I also need to renew my EAD very soon. My question is can I invoke AC21 at all for my new offer? Do I still have any hope to be adjudicated for green card? What are the implications or caveats? Thank you so much for your kind help!