You'll need to use a wide bladed screwdriver in the muffler slot to expand it out enough to break it free. There really should be a note in the instruction manual warning against fitting the muffler dry onto the stub because alloy against alloy like that will weld itself together. Take it apart, polish both surfaces then use a copper based anti seize paste from an auto parts store.

For the prop, the problem is that although the threads are 1/4x28 the machined area behind it is 7mm which is slightly larger than 1/4" (6.35mm). Try taking off the prop washer then use two propnuts as locking nuts (with a bit of clearance from the prop of course). Hold the outer nut with a spanner and then try turning the prop to get it to break free.