I would definitely load the stock tune before doing anymore troubleshooting.

Normally ECU issues that result in P0300 result in an new ECU, not just programming.

Fuel supply issues could be anything from pressure bleed-off to the pump going bad. Doubtful that its an injector issue because you have a better chance of getting struck by lighting while you're on your way to collect your mega millions lottery winning than you are to have multiple injectors go bad at the same time to cause P0300.

I'm not sure what the compression numbers are off the top of my head.... but that engine would have had to been neglected for a long time for carbon buildup on the valves to do this. Again, it would have to be multiple valves at the same time to foul all those plugs.

How's your oil level?