yes :)

Here is an explanation on μεταξύ vs. ανάμεσα. According to Filippos Petroulakis from Facebook Group Duolingo Greek Learners,

"αναμεσα is more physical, like "no one can come ανάμεσά μας", while μεταξυ is akin to "between you and me, Trump sucks"." "Of course στο μεταξύ means "in the meantime", which is completely different."

Let's put it this way:

• άναμεσά τους is accepted here because it's correct
• άναμεσα τους is accepted here because that was initially the only form in the course (for technical reasons which I can explain if you want) and can't be removed at the moment

The only correct form is ανάμεσά τους with two accents, because τους is unstressed and pronounced together with the previous word, and the "word" ανάμεσα τους would then have the accent too far back from the end.

So both forms are accepted in the Duolingo course for now but only one of them is correct.

It's one of the things we hope to do better in the second version of the tree.

That is not "the" correct solution (in the sense that there is exactly one solution and that is it), but that's one one translation that is currently accepted as correct.

I'm not sure whether it should be, since I think μεταξύ takes genitive pronouns, not accusative ones.

τες is the feminine accusative short-form pronoun used after a verb (e.g. an imperative) or a preposition that takes the accusative.

The second accent issue has been discussed throughout the course, in this very discussion, too (read mizinamo's comment above). It's something we've been taking care of for the second tree.