lisansw, this is very insightful information to many people here. I've researched this issue for over a month now and my understanding is exactly what you described. The most troublesome case would be RFEs on payslips.
I think there are two ways to deal with payslip RFE:
1. ask the old employer to issue an "intent of employment letter". This will transform the I140 case into future GC case. since the employer intends to hire you when your I485 is being approved, it is of couse you do not have payslips yet.
2. ask the new employer to reply the RFE with new payslips along with AC21 letter.
In reality, I am not sure if solution 1 will work. As respect to solution 2, I think there is a good chance uscis will take the ac21 case. Someone hinted me before that he/she consulted several atternies who were heavily involved in AILA. It seems uscis and AILA have had the consesus to allow AC21 on concurrent I140/485 which has been pending for more than 180 days.
Although payslip RFE is the toughest, problems still exist if the RFE is about W2 or employment verification. The W2 may reveal the fact that you did not get paid throughout the year, depending on the timing of the RFE and stop of receiving salary. Also the old company may not issue an EVL since you are really not employed with them. It's also tricky to get "intent to employ" letter if it's an layoff.
Of course, I get 70% chance not having to deal with any RFE issues.