You need to discuss this with a good laywer (Murthy or Khanna) immediately!!!
If you show the INS that 4 week old letter then this could be considered fraud, because you know already that you a going to get laid-off. This could spell trouble later (including revoke of GC). Would it be possible to ask the INS for a new interview date? For example, cancel the INS interview due to sickness (flu) or business travel? The new interview date might be off long enough so that you can find a new job and go with AC21. However, all what I am telling you is just my common sense. You should get in touch with a lawyer immediately! Good luck and please let us know what you decided (unless it is legally tricky, in that case don't tell anyone including friends and keep it only between you and your lawyer).