I think I-140 withdraw is different than h1 withdraw/ cancelation.
I-140 is for future job, so in case future job doesn't happen, I don't think employer has liability to cancel I-140 unlike h1 application, where they are libale to pay for retrun airfair or moving coast back to thier home country. so try to convice on that point. and tell them if they don't withdraw I-140, then in future if thier situation imporve you can go back to work with them. (one of my friend did this sucessfully) and if they are not doing great then they would like to find your own path instead of giving severance package and go through lay-off process. You can negotiate on various level. And anyway withdraw of I-140 involve more paperwork with INS with help of lawyer, so some employer won't bother and some might not know if they can withdraw. but I think any case good relationship may be the key if later stage complication created.
You are right about I-485 should be independet of I-140, but I saw some chat and case on murty that some case becuase of I-140 withdraw people have problem with I-485 application. so it's in gray area where it's in hand of fate or good lawyer. bascially Ac21 doesn't make any sense if I-140 withdraw after 180 days of I-485 filing create problem. but i can not tell you sure for either way. so i think it would be better to be safe and sorry.
Good luck.
JB