questions
Velma Winter gets into her old Datsun pickup and, after avoiding the demolition derby in the parking lot, discovers her right-of-way to the road is blocked by fleeing pedestrians.

Question 1: Does “right-of-way” in this case mean "a path or route”,not“the legal right to occupy a particular space on a public roadway” ?

At the Dome, the reunions have been ended by an apocalyptic party-crasher. Those on the inside have something more important than relatives to occupy them now: the giant mushroom cloud that’s growing to the northwest of their position, rising on a muscle of fire already almost a mile high. The first feather of wind—the wind that has sent Carter and Big Jim fleeing for the fallout shelter—strikes them, and they cringe against the Dome, mostly ignoring the people behind them. In any case, the people behind them are retreating. They’re lucky; they can.Henrietta Clavard feels a cold hand wrap around hers. She turns and sees Petra Searles. Petra’s hair has come loose from the clips that were holding it and hangs against her cheeks.

“Got any more of that joy-juice?” Petra asks, and manages a ghastly let’s-party smile.

“Sorry, all out,” Henrietta says.

“Well—maybe it doesn’t matter.”

“Hang onto me, honey,” Henrietta says. “Just hang onto me. We’re going to be okay.”

But when Petra looks into the old woman’s eyes, she sees no belief and no hope. The party’s
almost over.

Question 2: In “a muscle of fire”, “muscle” means “force, power or strength, right?

Question 3: The first feather of wind = The first breeze?

Question 4: hang onto me = hold onto me = follow me tightly?

Question 5: In "The party is almost over", "the party" refers to "the reunion" or "the Visitors Day", right? It has nothing to do with "let's-party", right?

Question

questions
Velma Winter gets into her old Datsun pickup and, after avoiding the demolition derby in the parking lot, discovers her right-of-way to the road is blocked by fleeing pedestrians.

Question 1: Does “right-of-way” in this case mean "a path or route”,not“the legal right to occupy a particular space on a public roadway” ?

At the Dome, the reunions have been ended by an apocalyptic party-crasher. Those on the inside have something more important than relatives to occupy them now: the giant mushroom cloud that’s growing to the northwest of their position, rising on a muscle of fire already almost a mile high. The first feather of wind—the wind that has sent Carter and Big Jim fleeing for the fallout shelter—strikes them, and they cringe against the Dome, mostly ignoring the people behind them. In any case, the people behind them are retreating. They’re lucky; they can.Henrietta Clavard feels a cold hand wrap around hers. She turns and sees Petra Searles. Petra’s hair has come loose from the clips that were holding it and hangs against her cheeks.

“Got any more of that joy-juice?” Petra asks, and manages a ghastly let’s-party smile.

“Sorry, all out,” Henrietta says.

“Well—maybe it doesn’t matter.”

“Hang onto me, honey,” Henrietta says. “Just hang onto me. We’re going to be okay.”

But when Petra looks into the old woman’s eyes, she sees no belief and no hope. The party’s
almost over.

Question 2: In “a muscle of fire”, “muscle” means “force, power or strength, right?

Question 3: The first feather of wind = The first breeze?

Question 4: hang onto me = hold onto me = follow me tightly?

Question 5: In "The party is almost over", "the party" refers to "the reunion" or "the Visitors Day", right? It has nothing to do with "let's-party", right?

Answer

1. The term "right-of-way" here is used unusually, in my experience.  In the UK, we say that a motorist "has right of way" when he or she has priority over cars coming from other directions, according to the traffic rules. In this example, the term is used to mean "path", as you correctly guessed, and so your confusion is understandable. I think that the author has been lazy.

2. "muscle of fire" is not a standard collocation.  It is likely that it means a powerful surging cloud of fire.

3. Good guess.  Again, it's not a standard term.  I would have guessed the same as you.

4. Good.

5. "The party is over" means that the good times have come to an end, and we must face a difficult situation.

The language is very expressive, and this makes it impossible to always guess the author's meaning correctly; but your instincts are good when it comes to guessing the meanings.  My advice is not to get stuck trying to understand the exact sense of every unusual expression you see.