-The cutoff date for May is Aug. 1, 2002, so if your father's sister filed the I-130 on July 31, 2002 or earlier, then the priority date will be current in next month, May 2015. Otherwise, it is not current yet.
By the way, your parents can not stay in US longer than the I-94 expiration date unless they file I-485 before the I-94 expires.
I have a few questions:
1. Can I add my Mom to my Dad’s 485 application and what kinds of documents are needed?
-Yes if the priority date is current. Need: copies of their birth certificates, marriage certificate, passport, visa page with the entry stamp and I-94 (print it out online).
(they are both here in US on a 6 months B2 travel visa which were planning to go back on 6/15/2015 – I94 date: 6/23/2015)
2. If we send in their 485, can they still go back to China around Sept this year?
-If they want to leave US before getting the green card, they have to apply for advance parole (file I-131 together with the I-485 filing), and use the advance parole to travel.
Or it’s better for them to stay here until green cards are approved which I think it will normally take 10-12 months (am I right?)
3. If they do go back to China after USCIS has issued a receipt and changed their status from B2 to AOS, do they have to obtain AP before their departure in order to guarantee a return for May next year? (Should I send 485 form with AP form together when we file next month?)
4. I know I’ve asked this question before but just want to confirm again, since my father’s sister applied for him but she’s not retired, can I add my name to his application as sponsor for financial purpose? (I’m greencard and my wife is citizen)?
-Yes, you can be the joint sponsor if your aunt does not have enough money for I-864. You need to file another I-864 as the joint-sponsor together with your aunt's I-864 (sponsor).
Thank you in advance for all your help!