Rotten transom? & check valves or stoppers for deck drain

What started as trying to address my deck drain holes letting water in when I have a lot of weight on the boat turned into a couple of shocking discoveries.

First, one of the through hulls on my transom well ('83 V20) was shot. I was checking it with my finger and it popped right out. Feeling around the inside of the hole with my finger found what can only be described as black group, certainly not wood.

Then later, when I lifted the port transom hatch and pulled out the insert, I was looking at the transom a nd discovered some failing fiberglass. An inspection with a screwdriver discovered, while mostly still rigid, a small area (1/2 inch) where it was soft.

Now, I could put a new through hull drain in the well and patch the cracked fiberglass, but you guys think I have a bigger problem? Transom still feels tuff and sturdy, but water has been in there for sure.

Also, was thinking about installing new deck drain fittings with plugs and leaving the Plugs in most of the time while running and fishing. Sound reasonable?

Finally, the forward bilge hose has come undone from the through hull fitting. Any way of getting to this from the inside?

Thanks,

Brad, aboard the Miss T