As per law this is very much possible what your lawyer suggests you!
BUT doing so will create problems for your GC process. Your intent is not so obvious that you will join A once GC is done. It is very very risky. One RFE after you are on EAD with B and you will be under tremendous pressure till you get your GC. Your interview will be certain too(very high chances).
====> I might join A before the approval of I-140/485. Don't know depending on the circumstances. But even with RFE on I-140/485 one can continue to work on EAD except for either I-140 or I-485 denial. What is the interview you talking about ? I didn't get it.
I will suggest try to put some pressure to continue your regular LC thru B or pay yourself for ad. Now all this is my personal opinion. Feel free to pick what your risk taking capability allows you.
