Question 1. Yes, cache is needed for all-flash configurations. It behaves a little bit different then in a hybrid configuration but you still need it.
Question 2a. The same link can be used for vSAN and other traffic
Question 2b. QoS best practises can be found in the vSAN Network Design Guide
Question 3. RAID-5/6 doe have a better storage capacity usage but also have a performance penalty compared to RAID-1
Question 4. Yes, it is possible to use the same link for SQL AAG traffic.
Question 5. Yes, this is possible. In a stretched cluster you can have a FTT up to 1.
Question 6. Yes, this is possible but not a best practice when a site failure occures.
Must of you questions are in detail answered in the following documentation:
vSAN Network Design Guide
vSAN Design and Sizing Guide
vSAN Stretched Cluster Guide
I also would advice to read the vSAN articles on Cormac Hogans blog; http://cormachogan.com/vsan/