Yes, everything you're planning is correct use 5 wires (3 phase, 1 neutral, 1 ground) to serve 277V loads safely, and avoid shared neutrals on lighting circuits.
thanks and regards
jack marvia
https://www.multispanindia.com
Yes, your assumption is correct—motor torque weakens during undervoltage, which can reduce speed. For PD pumps, this may cause stalling if the dip is severe. A 2-second restart delay might be acceptable, but if seal integrity is critical, consider larger motors, a VFD with ride-through support...