Wow, you have been taking my advice and gone way back in the old posts! Good for you.
The reason to make the CB hollow is the limit on total CB weight. This way, you can build a heavy (strong) skin w/o worrying about enclosing volume that leads to buoyancy (which would negate the effect of the allowed weight).
Looks like the drain is working as it's supposed to. Other than trapping water in the bilge, I'm not aware of places that need to be drained. Not all trapped water will cause damage; I had soggy foam that did not cause issues. I guess other than really tight spaces like inside the CB or inside the rudder (the latter only if cracked), there's usually room for the ice to expand.