My issue is the seized engine. What seized? Transmission? What was the squealing sound? Piston skirt to cylinder wall I'm guessing. The lack of oil would have caused an increase in heat which would have cause the skirts to expand and scuff, also the heat could contribute to the plastic pump gears taking a beating. You would think that the rod and main bearing shells would also see some damage but your oil analysis would have picked this up.
I remember my brother in law forgetting to add oil to the tank for his 2 stroke Johnson 70 horse outboard. He seized the engine. We had to wait a couple of days before it would turn over and then it would run fine. He did this 3 times over 5 years until the engine finally let go. Pretty tough engine. Looks like our Triumphs are tough also.
