Would "an-sin" be OK here ?
Yep, it's there!
I think it should. If it isn’t accepted as a correct answer, I’d report it.
Are 'an-sin' and 'ann' meaning 'there' interchangeable in general, or are there rules for when to use one or the other?
'an-sin' is like a specific there that you could point at, while 'ann' in this context is more like a state of being present similar to the word "out" when you say "the sun is out". Out where? What you're really saying is that the sun is present.