A new engine is going to be stiff. This is very puzzling. Sorry for you troubles. I'm suspecting either a weak battery or weak starter (or as some suggested it could have been locked up) Anyway, this is no consolation now but in the future if you run across this again, pull the plugs and try to rotate it backwards with a socked on the front pulley. You won't hurt the motor rotating it by hand if you don't used excessive force. I never asked if you tried to turn it by hand in the reverse direction but I guess I should have. There are ways to evaluate some of the internal components of an engine without pulling it but lets hope you never have to face that again. Even if the starter gear hung in the flywheel, you should have been able to rotate the engine by hand. Also I think there is a stub on the back of the starter for rotating it with a wrench. I've never had to but when I saw it, I supposed it was for rotating the starter back out of the flywheel if it got hung up.
JC