Hi, sounds almost impossible to believe, but of course i can look with a scope surely, have one at work i can borrow..As an old GTV-6 owner this sound like the timing belt has skipped a tooth or two... I remember many years ago helping a friend do the belt on his 12V 164... He didn't remove the cam cover to access the timing marks, he just made some paint marks on the engine because he found an "easier" way to do the belt on-line... Anyway, when he finally listened to me and tried to roll the engine over by hand it wouldn't turn very far before stopping but he could roll it 360 degrees backwards because the way the slack in the belt let the cams/valves and crank mesh. I told him not to make a habit of rolling that engine backwards but it really isn't as bad as everyone thinks if the plugs are out.
DON'T EVER ROLL A BUSSO V-6 BACKWARD WITH THE PLUGS IN, YOU WILL JUMP THE TIMING BELT OUT OF TIME! BAD THINGS WILL THEN HAPPEN IF YOU TRY TO START IT.
Having just done the timing belt and water pump (early April) on my 4C IMO turning the engine backwards with the plugs out isn't going to be an issue, there is plenty of contact around the pulleys and the timing belt tensioner will take care of the slack but I wouldn't make it a habit either.
Still, it is interesting that the engine will not turn the correct way at all. If the clutch were stuck or the engine hydro-locked it wouldn't turn in either direction... What comes to mind is that something broke almost immediately when you tried to start it the first time and now prevents the engine from rotating. But one would think if the engine will rotate backwards it should rotate forwards at least to the point where the damage causes interference.
I went back and re-read this entire post and IMO when you tried to start the car the first time something inside the engine broke. I don't know or why the engine will rotate backwards but not forwards but I'd guess that your manual manipulation of the crankshaft has moved the broken pieces around such that the engine will roll one way but not the other.
Just a guess is that a valve keeper failed and the valve dropped down into the cylinder. It hasn't fallen completely out of the guide but it probably isn't totally straight either. My thought is that when you try to spin the engine with the starter motor the valve locks the engine in place but turning backwards by hand is slow and jerky enough to push the valve back up into the guide. Forward is probably blocked by the damage to the valve and piston causing the broken parts to shift ever so slightly and restricting the engine's rotation.
I'd say go to Harbor Freight and buy one of their $60.00 borescopes and have look down into each of the cylinders. You may not like what you see...
cover is removed from the belt
A simple test is showing manually cranking the crankshaft, its has at any moment 0mm turning in the normal direction
Nothing is blocking the camshafts