For maximum safety, when you have multiple PSU for 1 rig, you have to connect all motherboard connectors, HDD, some GC and ALL risers on first PSU.
Secondary PSU should only be use to power graphic cards. If you split risers between PSU, you still have a risk...
No. A GPU and its riser should be on the same PSU.
Yes, agreed. Think about it. Also decent psus won't let you do this (tried years ago with an EVGA, wouldn't boot).