>The guy is still alive & breathing afterwards. Hence my
>confusion

No, that's Plainview. Just after the buckets hit Ailman, blood splatters everywhere, then there's a quick cut to Plainview breathing...

This is how it reads in the script:

>INT. SHAFT

DANIEL and AILMAN are in the shaft, working something. They are moving in and out of consciousness, but are somehow driving forward, fixing a part of the drill. OIL and MUD are pooled underneath.

Daniel hears something. Looks up.

ANGLE, THE TIMBER SUPPORTS THAT SURROUND THE WELL. something has come loose...

CU. DANIEL.
he looks at AILMAN.

CU. TIMBER
a very large, sharp piece, like an icicle, cracks in the support and comes flying STRAIGHT AT CAMERA.

DANIEL AND AILMAN. TWO SHOT.
THE PIECE COMES FLYING DOWN LIKE A DAGGER AND GOES STRAIGHT INTO THE TOP OF AILMAN'S HEAD, BLOOD PUSHES FROM HIS MOUTH AND EYES AND SPRAYS DANIEL.

HOLD, CU. ON DANIEL. In a state of shock.



As far as HW and Ailman goes, the first two times we see HW it's with Ailman. The first time is when he's walking around the site holding HW, the second time is when they're all around the puddle of oil and Ailman takes some and rubs it on HW's forehead, then kisses HW on the head. The next time we see him, it's as Daniel's looking at him, shortly after the Ailman accident.
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Magnificent Trident