Yes, an interlinear will correctly leave it out or put the implied or supplied word in brackets to show it doesn't exist in the original Hebrew text.
In this passage, no.
The wording is (as I recall) something like "For six-days God created..." and there is no sense of anything being missing. It is just how Hebrew and English differ in expression.
Yes, this is a good example of what I've described in various posts when telling people that chronological and tense details were not as important to the culture and language as Western minds assume is important.
I see your point about "a creation in six days" and "six days of creation" have different meanings in English---mostly in emphasis/focus. Yet, it is easy to imagine English Bible readers arguing over which of those two alternatives is appropriate, and Bible translators would be justified in arguing that some distinctions made in English attempts at rendering the Hebrew original simply wouldn't have occurred to the original Hebrew author and audience.
To illustrate that last point, here's an example I like to use from the Noahic flood pericope. Did the rising flood waters cover hills or mountains or both? That would seem like a reasonable question for English Bible readers because English strongly distinguishes between hills and mountains except that it is very hard to draw an exact line between the two. Yet, in the Hebrew text of Genesis, the word most commonly used doesn't distinguish between hills and mountains! (Thus, I sometimes like to informally translate the word as "high elevations", just to help students avoid the hill/mountain distinction in English which the Hebrew reader probably never paused to think about.)
I've sometimes asked students, "What is the difference between a hill and a mountain?" Everybody agrees that it is a matter of degree: mountains are taller and bigger than hills, but nobody knows the dividing line. Four hundred feet? Some people will say that mountains tend to have lots of exposed rocks while hills are soil covered and can be green and suitable for sheep and goat grazing. Yet, Americans from the eastern USA may say, "No! Our Appalachian Mountains are so beautiful because they are so lush and green. They aren't mere hills.." So because we modern day English speakers bring all of that "baggage" to the topic, we are inclined to want Hebrew writers to make similar distinctions---and we can get frustrated when they don't. And when I tell a global-flood-believing Young Earth Creationist that the Noahic Flood covered "all of the high hills" or that Noah's ark came to rest in the hill country of Ararat (and that there was no such place as Mount Ararat in the Bible and certainly no evidence of a landing in Turkey) with no explicit claim that all of planet earth's mountains were covered (such as 5+ mile high Mt. Everest), they can get quite angry with me. Of course, they may go really ballistic when I tell them that I believe the intention of the Hebrew wording of the only passage describing the magnitude of the flood waters specifies only about 22 feet in depth---which was quite enough to cover "the high hills", because the author had no reason to include huge mountain ranges elsewhere on planet earth and no knowledge of them. (Indeed, the Hebrew text speaks of "the entire ERETZ" which probably meant the entire land or country which Noah knew as his homeland. As I constantly remind everyone I can, ERETZ is OK to translate as "earth" as long as nobody automatically assumes that it means "planet earth.")
I went into that tangent for several reasons but mostly this one: The kinds of fine point distinction you are trying to resolve over the English word "in" is exactly the sort of issue which an English Bible reader can't resolve without relying upon Hebrew exegetes writing very detailed commentaries. Of course, even then you may find that the Hebrew text either (1) is "ambiguous" on the issue in question. (That is, it may be ambiguous from a western mindset or from the expectations of an English language speaker.) (2) The Hebrew author wasn't taking a position or even considering the issue, so no amount of dissecting of the Hebrew text may tell us how to answer the question.
Yet again we are facing the kind of issue that Mike Gantt is trying to tackle. He's not comfortable with the idea that Genesis 1 could give us a six YOM/day outline jam packed with all sorts of literary structural elements (like chiasmas and the 1/4, 2/5, 3/6 day patterns that Caspar H. described) without referring to an actual, literal six-day sequence of a conventional real week of days. Yet, all of these issues are related to an ancient culture looking at the world and expression itself in ways that are strange to us. We chronological and "literalistic" westerners assume the six numbered days must be a "real" workweek---especially when used as the basis of sabbath observance in Exodus 20:11. Yet, an ancient Hebrew audience would see emphasis in the "SIX days" of God creating, while many Christians today assume that the emphasis must surely be "six DAYS" (as in 24-hour days) or even "SIX DAYS" instead of "six days". (I think you see what I'm getting at by using upper-case for emphasis.)
I have several times in the past mentioned Carl Sagan's use of the "cosmic calendar" or "cosmic year". (I may be forgetting the actual term.Sorry. I can no longer trust my memories.) He used a literary device where the entire history of the earth was compared to a single calendar year of 365 days. He did it to help convey comparative lengths of time, such as no life appearing until the autumn and no mammals until the last days of December. Human history was a relative short blip of time in the evening of December 31, if I recall. Someone from another culture might read it sometime in the distant future and think "Those ancient Americans had no idea how old the earth is!"
I think also of a famous song from the Broadway musical _The Fantasticks". It used just a portion of a single year to reflect on one human life:
_Try to remember the kind of September
When life was slow and oh, so mellow.
Try to remember the kind of September
When grass was green and grain so yellow.
Try to remember the kind of September
When you were a young and callow fellow,
Try to remember and if you remember then follow._
_Deep in December it's nice to remember
Although you know the snow will follow.
Deep in December it's nice to remember
Without a hurt the heart is hollow.
Deep in December it's nice to remember
The fire of September that made you mellow.
Deep in December our hearts should remember then follow._
I've had some students strongly object to any assumption that Bible writers were being so "non-literal on important matters!" It never occurred to them that Genesis 1 contains six stanzas (one per YOM/day), each followed by a repetitious chorus: "And the evening and the morning was the Nth day." Does standard prose that is describing an historical event covering a single week usually involve a repeating chorus? (I've sometimes asked those who insist that Genesis 1 is just another "historical narrative" to point to another historical account in the Old Testament where a repeating chorus was used.)
At the very least, I want them to recognize that Genesis 1 has many very striking and atypical elements. Of course, I would like them to recognize that Genesis 1 (and Genesis 2, for that matter) could be based on oral traditions that were ancient even in Moses' day and that that ancient tradition may have been transmitted through multiple languages and cultures before the Hebrew writer(s) crafted it for their purposes as the ancient text we know today.
(I must leave for an appointment so I apologize for likely typos from lack of time to proofread. I see very poorly.)