If the clutch were seized, you wouldn’t be able to engage/disengage the transmission as you describe in your “highway test”.
When the Turbo failed did any bits get loose and get blown into the cylinders? That might cause pistons to jam.
As this stage you are into an engine stripdown anyway, even if you did get the engine to turn over. Obviously something is seriously wrong and even if the pistons or bearings did free up enough to allow the engine to turn over there would likely be damage to the bores or crank.
I would start by removing the cylinder head which might give you a better idea of what has happened. If nothing obvious is found the next step is to drop the oil pan (not sure if this can be done in-situ) and remove the crank bearing caps to see if the pistons free up or the crank will turn.