The rule doesnt say, as far as I know, "if the player elects to bump you have a duty of care to protect the opposition players head, unless you are also contesting the ball in which case this rule doesnt apply".
Its says "if the player elects to bump you have a duty of care to protect the opposition players head" period.
Mackay came in at a million miles an hour, saw, calculated, protected himself and elected to bump, plain as day. Sure he might have also been contesting the ball. Thats fine, just dont hit the head.
Running back with the flight of the ball, eyes on the ball, or two players who dont see each other as they have eyes only on the ball, are not electing to bump. Its an accidental and incidental contact. No penalty.
I remember Johnno electing to bump, hit a guy in the head with his rear end as he was bending over to contest the ball, he got done, whats changed?
The rules are there, it wouldnt change the fabric of the game, whatever that is, to enforce them.