Actually, there is a hole behind Luke, but barely visible, and maybe a bit too small given the massive rain of fire that poured upon him. And the hole's immediate surroundings are undisturbed, which can't be right given the cloud of debris created as the walkers fire.
The first shot:
I said increase firepower!
Just a tiny scar on the ...
Luke has "cut himself off from the Force". This apparently makes him invisible to other Force users like Rey, Kylo Ren and Snoke.
“I saw everything,” she said. “The island, and past it I felt the stars singing. I thought my heart would explode. But I didn’t see you. Nothing from you. No light, no dark. You’ve closed yourself off from the Force.”
From what we know, probably no one
Kylo Ren used ancient design for his lightsaber, back from the time of Great Scourge of Malachor. Kyber crystal in it was cracked, so excess heat was vented to sides as crossguard.
This of course begs the question why would he use cracked crystal. Kyber crystals were rare, but not that rare (Death Stars required lots of ...
Not a complete answer per se but a follow-up to Valorum's:
The First Order & Snoke had no real way to know where Luke had even started, and the Galaxy is a very big place. Consider this:
In ANH, Vader senses Obi-Wan while on the Death Star (within a ~80-km radius) and Luke when he's much closer during the Trench Run.
In ESB, Leia sensed Luke from ...