In my experience, when you have one or more siezed pistons, you get a tiny bit of movement in the crankshaft, due the tolerances of bearins, the the flexibility of the metal, when you get a siezed crank, you get absolutely no movement on that crank. I have also found that most siezed engines, its the crank, because all you have to do to sieze an engine, is not change the oil enough, the ports in the crank are fed from the oil pump, when you get dirty oil with sludge, it blocks those ports, dosent feed oil to the big end bearins, and they sieze, and the one that siezes first, is the one furthest from the oil pump, because its the one that blocks first, the main bearings, can be unefected, you just have to take the crank out, have it re ground, and oversize bearings fitted.
The fact you say thier is no movent on the crank, leads me to say, its the big end bearings, if it was the pistons, you would get a tiny movement either direction on the crank, might only be a couple of milimeters, but it will move, but with two big end bearings siezed, an engine wont move, In saying that, other things can sieze engines, when i was young, i worked in a garage, and got given an italian car to re build the cylinder head of, did all the work, put it back toghether, got a new head gasket (an italian one) didnt know that ALTO was italian for up, the head gasket looked the exact same in either direction, but for the word, ALTO on one side, but, when upside down, it had one small oil port blocked, which fed the overhead cam, took the car on a test run, the engine siezed solid, because the overhead cam siezed, which is why i say, it can be the cam, the pistons, the big end bearings, the main bearings, or, it can be a dozen things that are just jamming the engine, such as a siezed valve, stopping a piston from going up, a jumped push rod, jamming a rocker, a siezed cam follower, stopping the cam from going round, a siezed cam shaft.