The Indus Valley Script is a really tough thing to place because there's so little evidence to go on, and India has a particularly nasty problem with nationalistic cultural appropriation.

Deciphering unknown scripts is insanely difficult. In fact, all of the big scripts started with use of a bilingual inscription: Egyptian via the Rosetta Stone, cuneiform via a bilingual Akkadian/Persian inscription, and Mayan via a (botched) bilingual alphabet. There's nothing of the sort for Indus Valley Script, and the texts themselves are too short for even cryptology to be of much use. It's not even clear that Indus Valley Script is true writing as opposed to proto-writing, i.e., if it was capable of encoding full utterances within the language spoken of the time.

The link between the Phoenician abjad and the Greek alphabet is quite well-attested, not least of which is the order, shape, and phonetic correspondence between the letters is damn near perfect. Compare the correspondence of Phoenician and Greek versus Linear B and Greek, say. It takes pretty big blinders to say the Greek alphabet couldn't have come from Phoenicians.