I'd take a close look at the fan shear pin again and put a wrench on it or just pull hard on the fan to see if it spins. Quite sure I had something like this occur many years ago. The shear bolt was painted over therefore adhered to the shaft on both sides. It sheared but both sides looked fine. There was enough drag internally that the fan still spun up with no load but immediately would slow and stop under load.
This is where I would stay, cheaper and easier to double/triple check all shear pins first.