There is no such option on the web interface for Meta Stack Overflow site either. The flagging menu reflects the actual options available through the web interface.
You'll have to flag a post with the Other option (first screen) and request a moderator migrates such posts.
As for the top reason re-appearing at the bottom, that does look like a bug. When you have enough reputation, you get to fill in a custom off-topic because reason that would appear in that last slot, but if you don't yet then nothing should be listed there.