I have identified the problem with your technique...capsizing in the first place.

Seriously though, don't fill your mast with foam, corrosion and weight become issues.

Your mast sounds reasonably well sealed if it takes 5-10 mins to drain. You just need to get onto the hull earlier. Forget the spinnaker retrieval until you have the boat under control again (on it's side if need be) the retrieve kite from the hull wherever you can.