Why does the correct answer have "here".? "Віктора немає" mentions nothing about Viktor's location. It literally means "There is no Viktor". They may have a Igor there and not a Viktor.
I think "here" is in the answer because it makes sense in English. "There is no Victor" would have a completely different meaning.
Why this word order? Why not «Немає Віктора»?
Both word orders are fine, depending on what you want to emphasize.
Кого сьогодні немає? Немає Віктора.
Де Віктор? Віктора немає.
But it isn't a big deal. Right?)
Yes, this is correct. Немає Віктора is accepted.
No тут in your question, so here is not implied.
Some location is implied. The translations "is not here" or "is not there" are both accepted (or if not, they should be).
Could one also say
Віктора не тут....?
Віктор не (є) тут. Віктор is the subject in the sentence, which requires a nominative case.
Віктора не має (тут). This is a so-called subjectless (безособове) sentence, which puts Віктора in a genitive case.
I think you would say that as 'Віктор не тут'