Filing I-485 while an H1-B extension is Pending / Cross - Chargeability

[AM007]

Filing I-485 while an H1-B extension is pending 
Here's the background: 
1. I have an approved employment-based I-140 (just got approved last week) 
2. My Current H1-B expires on July 2018 (Maxes out for 6 yrs) and my employer is going to file for my H1-B extension next week based on the Approved I-140. The extension request will be for 3 years past my 6th year of H1-B. 
3. My PERM was filed in December last year so Priority Date is December 2017. I am from India so my 
PD will be 8-10 yrs from now. However my wife was born in Pakistan (which has a Current PD). 

Questions: 
1) Using my wife's country of Birth can I use the Cross Chargeability and apply for I-485? 
2)Do I need to wait for my H1B Petition to be approved before I can start on the I-485? What are the pro / con of Waiting for H1 to be Approved or Filing the I-485 while H1 is still Pending? 
Is there a chance I an receive an RFE on either the H1 Extension Petition or the I-485? 
3) Should I wait for my H1 to get Approved and then start I-485? 
4) Also do you know how long does it take to get the Green Card / AP after filing the I-485 ? 

Thank you so much.