Obscure Dash Usage on the PSAT

There are two rules that a student needs to know about the long dash (or em dash):\.

(1) One long dash is equivalent to a colon;
(2) Two long dashes are equivalent to commas around a nonessential (or nonrestrictive).

I have never seen a long dash in an ACT English question that didn't fall under one of these rules two rules. The College Board, however, likes to mess with student's (and tutor's) heads by finding obscure dash usages.

For example, question 15, from the 10-15-16 PSAT:

Stress levels, immune system strength, male fertility, and weight and cholesterol— 15 which all appear to benefit from the regular consumption of nuts.

A)  NO CHANGE 
B)  that all appear
C)  all appearing
D)  all appear

Answer choice D would work perfectly if not for that dash: "Stress levels, immune system strength, male fertility, and weight and cholesterol all appear to benefit from the regular consumption of nuts." Unfortunately, the dash is not underlined so there is no way to get rid of it. 

Applying Rule #1, the dash is equivalent to a colon, but the colon is supposed to be preceded by an independent clause. One of the most common uses of a colon INDEPENDENT CLAUSE-COLON-LIST, but here it's LIST-COLON-INDEPENDENT CLAUSE.

Is that even legit?

Apparently it is, according to several grammar websites I found, although none of them explained how this fit in with the rule that a colon must be preceded by an independent clause.

Another obscure dash rule came up in question #3 from from the 11-2-17 PSAT:

A turning point in the musical  3 genre; however, came with Richard Rogers and Oscar Hammerstein II's 4 Oklahoma!

A) NO CHANGE

B) genre, however;

C) genre—however—

D) genre, however,

My first instinct here is to eliminate both C and D because two dashes are equivalent to two commas around the nonessential "however." However, a semicolon may only be used between two independent clauses, and "a turning point in the musical genre" sure ain't an independent clause—with or without the "however." So I'm left choosing between two forms of equivalent punctuation.

The key to the answer lies in the following question:

The writer is considering revising the underlined portion to the following.

Oklahoma!—the first musical to feature a sophisticated story served by music and dance elements.

Should the writer make this revision?

It turns out that the author should make that revision, which leaves the following sentence: "A turning point in the musical genre, however, came with Richard Rogers and Oscar Hammerstein II's Oklahoma!—the first musical to feature a sophisticated story served by music and dance elements." 

Now there is a reason to prefer D over C. While the two dashes in C are equivalent to the two commas in D, using three dashes in one sentence is frowned upon because it may create ambiguity. So the fact that the answer to #4 adds another dash to the sentence determines the correct answer to #3.