Sentence Correction (SC) | Re: That the companies would merge was a certainty, though less clear were

Even though my initial answer was incorrect. I think I understand the solution now given byzhanbo .
And because a few of us are confused. Here's my take. Feel free to point out my mistakes if any.

"That the companies would merge"(Singular Subject) was(Singular verb) a certainty, though less clear were(Plural verb) "when the merger would take place, who among the top executives would be the chief executive, and who, if anyone, would voluntarily leave"(Plural subject - X,Y and
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