why "Mi örülünk, és ti? " is not a correct translation?
It is correct, and accepted now.
I just used te and it was not accepted
Why is " Mi boldogok vagyunk, és te " wrong in this case ?
No. That is a correct translation.
Um, why is this the preferred translation then? Is duo wrong here? (I wouldn't be surprised)
These word order issues are almost always about emphasis. Duo probably wants the emphasis to be on being happy, so boldogok immediately precedes the verb.
Isn't "Mi vagyunk boldogok, és te?" also a correct word order?
Is MI necessary?
I think so, given that it is being contrasted with "te".
Is "Boldogok vagyunk, ön és?" the way to do this if you were using formal?
It should be "Boldogok vagyunk, és ön?"
"Mi vagyunk boldogok, és ti?"? I'd have thought that "mi" and "ti" were being contrasted so "mi" should have focus. Also, I have a personal rule of translating "you" as "ti" or "te" depending upon whether "mi" or "én" also appears, respectively.
Is örülünk a valid word here?
Yes. And it is accepted.
(Mi) örülünk, és te?