When I had sunroof drain issues there was water sloshing around above the headliner. It is easy to check the drains with a glass of water and a dry floor. Pour the water in the roof and see if it comes out the bottom.
Bloc, I agree with everything that you say with one small difference. I believe that the cowl is the “heel”. Both my ‘08 LC and ‘14 LX have had water problems after the drains were cleared and I believe that the weak point is the cowl.
And, JST...