"LouC" Many thanks for suggesting I look for a melted flapper. I found exactly that. I owe you a beer. The next time my son goes back to NY, I'll have him look you up & tip up a few pints.
I removed the elbow and saw the flapper was gone but still had the pivot pin. Then I pulled starboard elbow and its flapper was still intact, so I removed it per the VP Bulletin. Next I pulled outdrive and found the port side flapper, melted to crap as you said it would be:
Good time to lube the U-joint spiders, glad I kept the stand:
I snaked a vacuum cleaner hose down both sides of the exhaust Y-pipe, don't think there was any debris.
No leaks in the manifold nor riser, so all I'm replacing is the lower boot and riser gaskets. Turns out the elbow which I thought was cast iron and blistered all the paint off, is actually aluminum. Same for the Y-pipe.
Transom bellows is still in good shape, same for the exhaust bellows, it was still getting sufficient water cooling from the starboard side, which didn't have any water blockage, and perhaps was getting even more flow from what wasn't going to the port side.
Thanks again!