Laid off on Dec31,2008.I-485 pending more than 180 days,I-140 approved,both priority & filling dates current.I like to invoke AC21 immediately from new prospective employer by submitting job offer without any pay-stubs. The reason is I intent to work with new sponsor only after I get my GC approved. In the meantime, I like to work for another small company on parttime basis with EAD.Please clarify me below Qs:

1) If for instance,USCIS might ask me why you should not work for new sponsor if you have valid EAD?
2) What is stopping you to start work with new sponsor right now as soon as invoking AC21 paperwork?
3) Why you have to wait till GC gets approved to start work with new sponsor?

Please suggest me how do I prove my job offer is legitimate or genuine from USCIS’s perspective?