I must disagree with the conclusion of the earlier answer to this question, to the effect that section 6.80 of The Chicago Manual of Style, sixteenth edition (2010) provides the relevant rule for a situation of the type that the poster asks about. That section, which is headed "En dashes with compound adjectives," is specifically concerned with compound adjectives linked to an open compound—most particularly a compound that is open because it is a proper name.
For example, in "World War II lyrics," the modifier "World War II" is an open compound; and in "Chuck Berry riff," the modifier "Chuck Berry" is an open compound. In such cases, according to Chicago 6.80, when a hyphenated element appears in addition to the open compound modifier—as in "post–World War II years" or "Chuck Berry–style lyrics"—it is appropriate to use an en dash rather than a hyphen to connect the single-word element to the open compound element.
Chicago 6.80 does include one example involving an unhyphenated set phrase that isn't a proper noun—namely, "country music" in the phrase "country music–influenced lyrics." It seems to me, however, that "country music" is unitary in a way that "expanding and contracting" is not. Beyond that, Chicago notes that the en-dash approach "is most helpful with proper compounds, whose limits are established within the larger context by capitalization."
What Chicago means is that, whereas we can immediately tell that "World War II" and "Chuck Berry" are the complete open compounds in those two cases, no such clear signal exists in the case of, say, "slow and mournful country music"—and as a result, we can't easily tell how much of "slow and mournful country music–influenced lyrics" is comprehended as a set phrase by the en dash between "music" and "influenced." The same problem in reverse haunts the phrase "and to the ever–expanding and contracting gulf between them": where does the authority of the en dash end?
For these reasons, I would look elsewhere for guidance on the poster's question, which involves how to handle a situation where a pair of compound phrases occur in a sentence, but the first element of one of the pairs drops out. Chicago discusses the reverse of this situation at 7.84, as follows:
7.84 Omission of part of a hyphenated expression. When the second part of a hyphenated expression is omitted, the hyphen is retained, followed by a space.
[Relevant example #1:] fifteen- and twenty-year mortgages
[Relevant example #2:] Chicago- or Milwaukee-bound passengers
but [relevant exception #1:] a five-by-eight-foot rug (a single entity)
Omission of the second part of a solid compound follows the same pattern:
[Relevant example #3:] both over- and underfed cats
but [relevant exception #2:] overfed and overworked mules (not overfed and -worked mules)
Relevant example #1, with both compounds stated in full, would be "fifteen-year and twenty-year mortgages"; relevant example #2, with both compounds stated in full, would be "Chicago-bound or Milwaukee-bound passengers"; and relevant example #3, with both compounds stated in full, would be "both overfed and underfed cats" because the first instance of "year," the first instance of "bound," and the first instance of "fed" in these examples drop out, Chicago recommends leaving a letter space after "fifteen-" and "Chicago-" and adding a hyphen to "under-" to signal the omission of the word that appears after the hyphen in the subsequent compound in the same phrase. It does not recommend replacing the first hyphen with an en dash in such situations.
The poster rather arbitrarily asserts that there are only two options for punctuating the phrase of interest:
a. and to the ever-expanding and contracting gulf between them.
b. and to the ever-expanding and -contracting gulf between them.
We see very quickly that option (b) above runs afoul of relevant exception #2 in Chicago 7.84: "ever-expanding and -contracting" is functionally indistinguishable from "overfed and -worked mules," which Chicago explicitly rejects. But it doesn't follow that option (a) is the default winner. The problem with option (a) is that it shows no sign of having dropped a second instance of "ever-" from its wording; as far as readers can see, "contracting" is not a compound modifier at all but a simple one-word modifier. This is clearly not what the poster wants to convey to readers.
Fortunately, the poster has yet another option available. It is to take as its model relevant exception #1 above and hyphenate the entire string of modifiers in the compound. In this regard, "the ever-expanding-and-contracting gulf" has a lot in common with "a five-by-eight-foot rug." That is, it expresses in shortened form the idea "the ever-expanding-and-ever-contracting gulf," just as relevant exception #1 expresses in shortened form the idea "a five-foot-by-eight-foot rug." That gives us this Chicago-approved treatment of the phrase:
and to the ever-expanding-and-contracting gulf between them.
The poster says "As a former journalist, my instinct is to keep punctuation to a minimum," which may lead to unhappiness with the suggested handling of this phrase. But the way to resolve that conflict between personal preference and Chicago rule isn't to ignore the rule (if you are bound to follow Chicago, as the poster is), but to alter the offending phrase. Here the poster can easily accomplish that result by replacing "ever-" with "endlessly" and getting rid of all the hyphens:
and to the endlessly expanding and contracting gulf between them.