You could also try swapping injectors between two cylinders to see if the problem moves.

Also, most of the story sounds pretty normal, and your debugging is great. Except what about this: "2 plugs cores were burned and the tips of the plugs were burned right off" ???

I'd try to figure out an explanation for that one and not just hope it does not come back.

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2001 X5 3.0i, 203k miles, AT, owned since 2014