請問小白兔，pjing,或其他大俠: short visit to China before AP expired

請問小白兔，pjing或其他大俠: 請教一個短期回國(business trip) 問題

我的情況: 140 approved more than 2 years ago, 485 pending more than year and half, have a vaild AP until 12/20. Has not renewed my AP yet. H-1B visa stamp valid until 3/2010

My company wants me to travel to China for a week or two this December
問題:
1. Is it mandatory to have a valid AP to temporarily leave US while 485 is pending? I am going to use my H-1B to come back instead of AP since the stamp is vaild until 3/2010. I actually just used H-1B to come back the end of November.
2. If I arrange my trip return date before 12/20, I should be fine right?
3. Is there any issue if I renew my AP after it is expired? Of course, I will return to US before 12/20 and stay in US while AP renewal application is pending. (the renewal application might be submitted after 12/20)
4. In my AP page, it says valid prior to 12/20. Do it mean before and include 12/20?

Thank you all for your kind information.