Translation:The green dresses are hers, and all of them are beautiful.
Would it be appropriate to say "The green dresses are hers, and are all pretty" instead of "...and they're all pretty."?
It's probably not just a coincidence. That is a very common way to create new nouns in Hebrew.
What's wrong with"The green dresses are for her, and all of them are pretty" ?
"For" isn't the same thing. I'm learning too, BUT in English "for" implies that they aren't hers yet. Maybe they are a gift, or were created with her in mind, or they will be hers once she picks them up.
Is it wrong to say, "and they are all pretty" instead of, "and all of them are pretty?"
See my previous comment. It's a fine translation.
The designers of this sentence were probably looking for a structure that directly parallels the Hebrew, with כולן translating directly to "all of them". (Either that, or they just didn't get around to coding multiple correct answers.)
I wish they would directly parallel the words in all their sentences. Pretty is יופה , not only "beautiful.