Look down the hole where the starter sits and check for any loose bits of metal you might need a torch inspect for any signs of metal.
So far from my understanding there is 12V to the starter from ignition key right?
(IF NOT POSSIBLY SOMETHING IS EARTHING OUT SO YOU DON'T GET ENOUGH CRANKING AMPAGE POWER TO START)But its not spinning when its mounted to the block? Right?
When its off the car it spins nice and freely when 12V is applied right?
If all conditions are met then something in your block is stuck I have a feeling the cog that the bendix sits on is jammed somehow the only situation I've come across is when the shaft of that cog has broken and now jams up the whole cranking shaft. Which is why I want you to bum start it becuase if it starts without the the starter the car is fine and the starter crank shaft has shat its self somehow?
PS try disconecting the alternator cable to battery and see what happens!