Senario 1: peer-link down, keepalive still working
Q1: What are the reasons for Secondary switch to shutdown all its vPC member ports?
Just to avoid a split-brain scenario, because you have 2 switches as the same to the rest of the network (STP point of view).
Q2: When vPC member ports on secondary switch are down, PC-B and PC-C will lose 50% of uplink BW, and PC-D will lose connection to PC-A/B/C. How these disadvantages are concerned by Cisco?
PC-D will continue to reach at the others hosts by the uplink in the secondary switch if layer 2 segment still exist. The documentation is clear about this issue: In a vpc topology maintain orphan ports isn't recommended.
Q3: What are the detailed reactions to the comming back of peer-link. vPC domain automatically recovers without human intervention?
Yes, since the vpc peer-keepalive still working at, this a default behavior.
Q4: Why not switch to split-brain scenario (both switchese claim the primary role and working properly) in response to peer-link failure, as this ensures 100% utilization of uplink BW.
A split brain scenario must be avoided at any cost, to keep the network information consistent.