Can you also say: "הבננה נמצאת ממול האבטיח"? What is the difference between מול and ממול?
That's interesting, I don't think I've ever used ממול, and I don't recall any rule about how to use it. It seems to me it would either be used to say something like "across from here", החנות נמצאת ממול, or "across from something that was just mentioned, וכשאתה רואה את הבנק, החנות נמצאת ממול, or else it requires ל, so in this example, הבננה נמצאת ממול לאבטיח. But these are just my thoughts, if anyone knows anything more definite please correct me.
I second the feeling that ממול feels like talking about something that was already mentioned, but like you, it's not more than a feeling of a native speaker.