Madara was the first to combine senju with uchiha. Those are the recquirements for izanagi as told by all their secrets begotten from the ancient stone tablet.Nope it was never stated that madara was the sole user, I'll need mangafact for that, it was stated that it was an uchiha kinjutsu, nobody said (BZ and the rest) who invented it, we just know the uchiha's did, you'll have to give us proof that uchiha's before madara did'nt use izanagi also, as nobody said madara invented the technique
During Itachi vs Kabuto kishi rewrote his own story to make ALL uchiha use Izanagi just do he could create izanami.