Hi BD, plumbing is not my long suit but I'll take a shot.
The seat may have calcium build up or is corroded, not allowing the rubber valve seat washer to sit flat. You are a trouper, drilling out the set screw. I would have replaced the faucet at that point.
Register for free to join our community of investors and share your ideas. You will also get access to streaming quotes, interactive charts, trades, portfolio, live options flow and more tools.