Wow. you're line of thinking was the same as mine, now I'm as confused as you.

I'm thinking that the flex plate is no longer secured to the crank - no idea how, but it makes sense out of everything EXCEPT the fact that when you turn the crank, the flex plate moves. However - I think this is because moving the crank has more resistance then moving the flex plate. when you turn the crank by hand, the flex plate still has some frictional contact (at it's broken junction?) with the crank, so it moves because there isn't much resistance to the flex plate moving. But doing the opposite, ergo moving the flex plate, there is not enough friction between the flex plate and the crank to overcome the resistance of the crank moving the pistons up and down, the accessory belts, etc...

I seriously think you will have to unbolt the tranny from the block, and see what's happening where the flex plate is attached. It will be interesting to see what happens

I've never heard of a cracked crank in a 420a - so I'm really thinking it's at the flex-to-crank junction.

2gnt: '99 RS-T, killed by a toyota, pending rebuild...
Daily: Volt
Daily #2: '99 EVG ebike- STOLEN by PEDRO