As for the channel in back of the pylon, I wouldn't silicon it although a lot of people do for some reason. You want water that splashes up under the pylons to drain out. If you seal it up the water will go down the vent tubes into the hull instead. I don't think it has a designed purpose though. It is just the way the metal is extruded.
I'm really confused by this statement. Could you clarify? The channel extends all the way down the hull so if you don't seal it it seems you are assuring water will work its way down into the hull. I also don't understand how this affects the vent tube that is inside the pylon and then inside the foam and then covered by the corner casting. The channel being filled with silicone or not won't affect the vent tube in anyway that I can see. Just trying to understand your statement as I'll be putting my hulls back together in a couple of months.
Btw I think it was very clever of hobie to simply use the same tubes for the mast as the pylons. At least I assume they are based on size and shape. Very cost effective.