There were two distinct examples of this word's use seen in the opera. One of them was obviously detailing how a group of warriors was killed one at a time: ngIq nuv luHoH "they killed the warriors one by one." The other was a thrice-repeated ngIq tonSaw lo' 'ej... "in one single move, he...", making it clear that each of several results was the result of the same action. I surmise that ngIq by itself could mean "the one and only". Again, I didn't copy the lyrics, and what I have written here is probably not exactly what was on the pages we inspected.

For the archive: this is printed in qepHom 2017, page 24.
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ngIq can indeed carry the "one after the other" or "one by one" meaning.  So ngIq mIch wIHotlh could mean "we scan each individual sector one after the other" or "we scan the sectors one by one."

Hoch does not have this sequential meaning. Hoch mIch wIHotlh is simply "we scan each sector".

Hoch focuses on things as members of a group; ngIq focuses on the individual things. It's a subtle difference, but a difference nonetheless.

Both Hoch and ngIq could be translated with "each"; Hoch, but not ngIq, could be translated with "every."  ngIq could also be translated as "single" or "individual" but the implication is that there are or could be other similar things as well (even if they don't matter for what is currently being talked about).  For example, in paq'batlh, the phrase ngIq tonSaw'is translated "one single move." The implication is that whatever is going on here could have been or might have been accomplished with several moves (presumably sequentially), but one was enough.  Saying wa' tonSaw' would not get across the idea that more than one move was possible but was not needed and did not occur.

Sometimes ngIq and Hoch are interchangeable. For example, a sentence in Monopoly is ngIq gholvo' wa'maH QaS yItlhap "Collect 10 forces from every player." Chances are, you'll take the forces from each opponent one at a time, one after the other (so ngIq is used appropriately here), but collecting the forces sequentially is not important as far as the game goes (so Hoch would have been fine as well).

ngIq X is considered inherently plural (like cha and some other words) since it refers to more than one X (if there were only one X, you'd simply say X or wa' X.) As such, it is grammatically singular, so wIHotlh in your sentence is correct; DIHotlh is wrong.