The backplane draws power across all the molex connectors that are plugged in. See
This is all discussed in Molex Clarification. The TL;DR seems to be that the potential issue is not the PDU, but the spec for the amp draw across a single molex connector. A single PSU to 4x molex puts you at or over that. You are better to spread the load over at least two PSU ports.
Doesn’t matter, see #1.
This would be the preferred minimum setup. See #2.