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.
Disagree; 10 minutes of water, is a LOT of Water to try and lift up when it's at the bottom of the mast. 10 minutes might be 10-20 litres which is 10-20KG just where you do not want it.
I would focus on fixing the leaks
Fix all known leaks and then put it back in the pool to test it. Repeate until all found.
I had a very leaky Inter 17 mast and it took me ages to find them all.
Also, any leaks up the top of the mast will be under some pressure when you tip the boat in. a couple of feet of depth will force water into even the smallest hole!
