The Dragon’s Tooth
This is the first book in the Ashton Burials series, by N.D. Wilson. The protagonists are Cyrus and Antigone (not the ones from the play named after the latter). You can see the silver snake bracelet, mentioned in the question, on the cover. It’s actually an actual silver snake (worn as a bracelet):
Every head in the diner turned, but Cyrus didn’t notice. He was
unwinding a snake—now visible—from around his wrist. Slender, silver,
smooth, it twisted around his fingers and slid its own tail into its
mouth. As it did, it disappeared.
Horace chuckled. “Little Patricia, I am very glad to see you. Or not,
as the case may be.”
One of the protagonists, Cyrus, starts off the book by making waffles:
“C’mon,” Cyrus said. He picked up a battered metal fork and rapped the
waffle iron. “Seriously, how long does it take? Turn green already.
It’s just a waffle.”
The light reddened slowly, and then faded to nothing, reddened again,
and then drifted away, reddened—
“Fine,” Cyrus said. “You’re done.” He popped the seal on the iron and
lifted the lid. The waffle seemed solid enough, at least where it
wasn’t sloppy with chocolate bruises. Cyrus forked it loose, slapped
it onto a plastic plate, and grabbed a stack of paper napkins.
Whistling, he hurried out toward reception.
We have a literal lightning bug:
Cyrus’s feet began to tingle. With a pop and a crackle, the lightning
bug launched and landed and launched again. Blue electric arcs trailed
from its abdomen and flicked between its wings as it circled,
bumblebee-heavy.
There are many people who can’t die (immortals of various sorts being a prominent part of this series), but the one who lives in a place with spiders is probably Nolan:
“What’s not safe about it?” Cyrus asked. “What are we talking about?”
A chuckle reached them, doubling and tripling off the angled walls,
and then reaching them again. “The Whip Spiders. Why do you think I
have this place to myself?”
“They’re still here?” Antigone scanned the floor. “That was over
eighty years ago.”
“It was,” Nolan said. “Whip Spiders can hatch many young in eighty
years. Stay on the paths.”
The immortal more often associated with spiders is Arachne, though.
There are several characters who could be referred to as “demons,” but a “demon guy with guns” who is killed by Patricia probably refers to Maxi:
The silver snake, suddenly as thick as his arm, struck straight for
Maxi’s face. Shocked, releasing Cyrus, Maxi staggered back, knocking
the first strike away. But the snake was still growing. Patricia slid
down Cyrus to the ground and reared up after Maxi, chest height,
hissing like a monstrous silver cobra, emerald eyes sparkling with
wrath. Maxi raised his gun and fired as Patricia struck the barrel,
taking the fireball down her wide throat. The snake’s thick body
bulged, glowing orange, and then a pillar of fire exploded back out of
her mouth and up Maxi’s arm.
He’s not really killed by this, though, being transmortal; it takes the titular Dragon’s Tooth to do him in.
