"The woman wiped her dagger on the rider's boot then immediately moved to the entrant behind Jack who stepped over the rider's body as the line moved up."
First off, let's fix that "then" problem.
The woman wiped her dagger on the rider's boot. She immediately moved...
You need a comma after "who" whether its antecedent is "Jack" or "entrant," because the clause beginning with "who" is nonrestrictive in either case. Saying "the entrant behind Jack" identifies the person. Saying "Jack" also identifies the person. (What would not identify the person? Oh, saying something like "the man." Then the clause would be restrictive: it would have the job of identifying a person who hadn't been identified already. It would
restrict the meaning of "the man" to one among all possible men.) Mastering the difference between restrictive and nonrestrictive clauses is important for deciding where commas go. You don't need to label them "restrictive" and "non-," but you need to know which kind you're dealing with.
If there's a commma after "who," then does its antecedant become the entrant behind Jack?
No. The reader still won't be able to tell who stepped over the body.
Bottom line: what changes, if anything, by inserting a comma after "who?"
The punctuation becomes correct, but the meaning doesn't change. The referent of "who" remains uncertain.
