"There will be aerodynamic rotor-to-rotor interaction on all craft with twin rotors (inc. main+tail)"
Feathered retreating, outboard advancing, (FROA) intermesher will be no worse than a conventional.
"On the Interleaving configuration, the greatest vortex coming from the retreating blade will be at the tip."
Depends on twist & taper. My real objection is the drivetrain complexity. On paper 4x4 cars gives the best performance, but few are sold due to the cost.
"I don't understand your disk loading concern."
A single rotor the same diameter as the interleaver will require less power/weight (approx 1.4 times if tip passes close to hub). F.R.O.A. intermesher gets close, by having a compact rotor system.
I see the F.R.O.A. intermesher as a bit like a fixed wing, except the wing moves forward to meet the air at the optimum velocity. The retreating bit is just a convenient means of returning the blade. When i say feathered retreating i mean twisted and pitched so that blade is at zero AOA to inflow (at 270'), hence no vortex to spill off.
The trouble really is Dave that we are both disputing respective speculation. The only way to resolve the optimum solution is going to be a test rig, some smoke, and a strobe...
Mart