"Niemal" isn't a word, but the "-s" on "niemals" is that same genitive ending as on "nichts," just added to a combination of the words "nie" and "Mal" rather than one word "niemal."
So much for my theory. :) If the exercise comes by again, click the "Report a problem" button, and describe the problem under "other problem" (you don't have to lose a heart for this! )
Another time, if you think your "incorrect" answer is probably right, go ahead and report it -- the Duo team will sort it out.
I was told that when discussing times in the evening I "should" use neunzehn Uhr as opposed to sieben Uhr. Is this an overly rigid recommendation? I imagine when you call a restaurant to make a reservation, they know when you say sieben that you mean in the evening, but I also know that many people don't operate on a 12 hour clock in Germany, so is one more correct than the other?
"approximately" = "around". No need to use both. Also, whichever you choose, it usually comes immediately before "seven":
"at approximately seven", not "approximately at seven." In this case, the German "um" means "at."
So: "We come at approximately seven."
If you use "around", you don't need to use "at:"
"We come around seven."
The last example is probably the best option. I would probably say "We come around seven."
Curiously, in English there is a phrasal verb "to come around", so you could say, "We are coming around at approximately seven o'clock," but it would not mean the same thing as the German (whose meaning has already been stated correctly by CraigGirdlestone and thetimesurfer).
That sentence has a different meaning than the German sentence.
"We are coming by seven o'clock" means we may come significantly earlier than seven o'clock, but we will certainly not be any later than seven.
"Wir kommen um etwa sieben Uhr" means (as far as I know) that we may come at 6:45 or we might come at 7: 20, but it will be approximately seven.
I think the best translation for this sentence is "we are coming at around seven o'clock."
There's nothing wrong with using "at", provided you use it a little differently.
Prepositions are often one of the hardest things to learn in a new language, because which preposition is used with which verb or in which phrase often seems to be quite arbitrary. Why, for example, do we use "to go off" for "to explode"? Why not, "to go out"? And for that matter, why do we say, "to go out" not "to go in" when talking about a fire being extinguished?
"Um" is frecuently translated "around" as in "um der Welt" (around the world), but it just happens to be the preposition of choice when talking about time, where the English preposition of choice is not "around," but "at."
Um acht Uhr = at eight O'clock
Now our English preposition "around" has several meanings too. If the dictionary hints show "around" as a translation for "etwa" that's not referring to "around" as in "around the world" (that would be "um"). "Etwa" means "around" as in "approximately".
So, all the words in your sentence are correct; you just need to reverse the order of "around" and "at".
"We come at around 7 o'clock".
If you don't change the word order, "around" is interpreted as part of the phrasal verb "to come around" . . . which means something different from the simple "kommen", and also means it is no longer translating "etwa", whereupon you gain too much precision. . .
. . . Aren't prepositions fun . . .
"Almost 7:00" means it isn't yet 7:00. It could be 7:57, or 7:58, or 7:59, but it cannot be 5:02.
"Around 7:00" means it is approximately 7:00. It could be 7:54, or it could be 7:00, or it could be 7:06, or any time in between.
Of those two, "etwa" means the latter, as far as I can tell.
You can compare some example sentences here:
Basically, "etwas" means "something" and "etwas" means "approximately".
(Just a warning, the dictionary bab.la has the weird quirk that it sometimes highlights the wrong word, and thus lists an invalid translation. So you can never just read down the list of translations and trust them, you always have to look at the examples sentences. But if example sentences are what you want, it's a great dictionary.)