Regarding your update: close, but not exactly.
IPN will return VERIFIED or INVALID depending on whether or not it recognizes the POST data you're sending as having originated from PayPal.
So yes, if you browse to your IPN URL directly and it directly POST's no data / dummy data to PayPal, then yes, we won't recognize it as having originated from us and we'll return INVALID.
For example, this link will return INVALID: https://www.paypal.com/cgi-bin/webscr?cmd=_notify-validate&dummy_data=true