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.
No, sorry, everything related to motherboard must share the same ground. By plugging risers on different PSU you have a chance to join 2 different ground and make your motherboard break down. In worst case it can start fire. Plugging only graphic cards on a second PSU won't make any problem.
I won't take the risk to see my house burn just for bad plug on a rig...