Yes, 'a flu' would work as a translation, although it's not a common phrase in English, but as it's a grammatically correct translation it's been added now.
'They have flu' is the preferred answer here, any others are alternatives.
'got' (or gotten in America) is something that many native English speakers add to 'have' even though it adds no extra meaning to a 'have' phrase.
For me, an American, "they have got the flu" and "they have gotten the flu" are subtly different. The former is identical in meaning to "they have the flu". The speaker is simply asserting that "they" are currently sick with the flu. The latter, "they have gotten the flu", is the same as "they have caught the flu". They are currently sick, but the notable thing is the transition from wellness to sickness rather than the current state of sickness.
The notes in Duo are the same as mine from Cardiff uni & Elin Meek's book: arno i ; arnot ti; arno fi (fo for N Welsh); arno hi arnon ni; arnoch chi; arnyn nhw
I can't find anywhere in the notes where it uses "arnon" as the 3rd person plural. It is clearly 1st person plural (we form) with ni.