I guess, the meaning of the hungarian sentence in English is the question: Shall we sit on those green chairs? But there is no shall in the hungarian sentence and it might confuse.
Yes, I think your English translation gets the meaning of the sentence exactly right.
I think the "shall we" version would be more closely matched by "... üljünk?" Which is the imperative version of the verb.
But it is up to the context to pick the best version. We could also say:
"Odaülünk azokra a zöld székekre?" - this is kind of an invitation or suggestion to go and sit there. So, a good "shall we".
This does not make any sense in english. Are we pointing at chairs and asking the question? then the question should be "do we sit onto those ..etc" or "do we sit down on (onto) those...etc"
Sure it can make sense. Imagine you are in a photo session with some other people and tour asking the photographer where you should sit.
It is so frustrating to repeatedly give answers that should be accepted and then have them rejected. I used "onto" instead of "on" which is technically more accurate than the accepted English translation since it actually implies movement. I wish there were an option where a user could override Duolingo's evaluation as to whether a translation is good or not.
You can (and should) report if you give a translation that is not accepted but ought to be. However, "sit onto the chair" sounds weird to me in English. A much more natural way of saying it is "sit down on the chair" (which was also rejected, and reported).