Can a broken seal in the exhaust where it connects to the manifold cause fuel to go in the oil and prevent the car from starting?

Can a broken seal in the exhaust where it connects to the manifold cause fuel to go in the oil and prevent the car from starting?
No. The exhaust manifold only carries burned exhaust fumes from the engine to the rear of the vehicle. It has nothing to do with the oil or fuel system. It also cannot prevent the engine from starting. However a clogged catalytic converter can cause the engine not to start. Fuel in the oil is caused by a leaking injector on a FI engine or a bad needle valve and seat of an carburetor engine.