...1. How much margin is there below the limit before the rotor stalls on say an R44 at 2,500 lbs on an ISA standard day at sea level?...
General,
The manufacturer does not have to test the helicopter to any specific margin beyond what is published in the operator's manual or what is required by the certification authority.
"How much margin"?, I guess some of the test pilots and engineers may know but it will pure guess work from our part.