The only thing from all those pictures that seems "off" at all is the #1 exhaust rocker is more worn than the others. But not enough to indicate catastrophe?
Many of the rocker cover screw holes into the head look pretty buggered up, Nik, you likely need to tap those holes carefully to clean them.
While you're out today in the light, kick the motor slowly and watch the cam rotate. See if there is any point at which it feels crunchy or hung up. If so, pictures. Without the rocker cover on, your valve train is disconnected, so those will need to be investigated manually.
Look carefully at the retainers and seats of the springs. Anything out of the ordinary?
And a last idea, what's going on with your exhaust? Any chance you've got a wicked hole or gap near the head stud and it's come loose? (Could explain the loud noise, which may be unrelated at all to the oil light).
I'm with Scottly, this may all be good news in terms of nothing internally wrong, something just funky. Could turn out to be more an investigation gone cold. But still got look some other places.
