"I am not going back to the US yet."
I would say it shouldn't be there at all because the real topic of the sentence is you not going there not America as a location. But can't know for sure
I constructed the sentence without the は and it was accepted as of 17/12/18. まだアメリカへかえりません。
Why is it "not going" if it is "かえりません"? Like how come it isn't "I will not go"?
Why do they use both he and ha/wa? I thought it was either one or the other.
It's the contrast form. It makes most sense in a context.
Edit: Maybe it being negative means it makes sense to use the contrast form inherently, but I wouldn't know.