Hello,
The lid damping factor is an empirical value - generally we recommend that you adjust the damping factor so that the simulation results match the experimental data. That being said, damping factors of 0.5 and 1.0 are quite large - we would usually expect values in the range [0.01 - 0.1]. It may be useful to run the hydrodynamic analysis with the External Lid surfaces suppressed, to gain a better understanding of their effect on the solution.
The Hydrodynamic Diffraction Structure Selection allows you to collect structures into one or more Interacting Structure Groups. If all of the structures are grouped together (this is the default behaviour), then the radiation and diffraction effects of the subsidiaries will affect the hydrodynamic properties of the main platform, as you observe.
I hope this helps.
Cheers, Mike