Your lawyer is right, if the old employer agrees to sponsor the GC application, you'll be fine, with one condition that you have to work for this old employer after your GC approved for a reasonable period.
One more point, you can't use any EAD, AP based on this 485. This 485 is not covered by AC21. This 485 is for future employment, that is, you'll work for this sponsor after yr GC approved.