My wife's Subaru had a similar problem. The front pulley bolt had come loose or something then it beat the key out until there was a really wide keyway in the pulley. I got a new pulley from the junk yard and it's all good again now. Loctite is your friend

If you're running 5w30 or 10w30, it should have oil pressure almost immediately upon starting and not drop unless the engine is in really, really bad condition. Your oil light would be coming on in that case. The best way to tell bottom end knock that I learned as a kid is to rev the engine a little and listen for hammering as the rpm's come back down. The theory is that if a rod journal is hammering, it'll hammer more when rpm's are normalizing than when under load.

Other things to check are torque converter bolts, flexplate bolts, and fan clutch. I just noticed last week that the service manual says never to reuse the torque converter bolts. I'm not sure why that is, as it's always worked for me in the past. I'm ordering new ones when my engine goes in, though.

I know that feeling of frustration.