Fuse is a safety feature. In the event of an electrical fault, where there is a short circuit current which exceeds the fuse rating, the fuse will melt and break the circuit. (fuse is blown) This will disconnect the appliance from the high potential of the live wire. The appliance will not become live and it will not be 'live' at 240 V.

Changing the pin will not change the voltage required by the appliance.

The wire used here is not the typical copper wire. It is nichrome wire which has much higher resistivity compared to copper. Hence, for the same length, the resistance of the nichrome wire has higher resistance. When the current flows through the nichrome wire, energy is transferred electically to internal (thermal) energy of the nichrome wire. The wire becomes hot so that it can cut the styrofoam with ease. This is the same concept for heating appliances like kettle etc.

A partially inflated balloon is placed inside a sealed container as air is pumped out of the by syringe. As the pressure of the container is reduced , the volume of the balloon increases . The pressure inside the balloon decreases . Note that the pressure inside the balloon is not equal to the pressure of the container . The number of air molecules in the balloon is fixed. As the pressure of the container decreases, this creates a pressure difference inside and outside the balloon. This causes the balloon to expand and its volume to increase. The number of air molecules per unit volume inside the balloon decreases, hence pressure inside the balloon decreases. But as the balloon is elastic, the wall of the balloon is stretched. Hence the pressure of the balloon will be greater than the pressure of the container. Refer to the video below.

When the pump is switched on and the air in the jar is gradually removed, the pressure in the jar decreases. There will be fewer air molecules per unit volume in the far. Hence rate of collision of the air molecules with one another and with the wall and hulk will be reduced. As pressure P = F/A, the force acting per unit area decreases, the pressure decreases. In the marshmallow, there are pockets of air at normal atmospheric pressure initially. As the pressure in the jar decreases, the pockets of air in the marshmallow expands due to this pressure difference. Hence the hulk expands and its volume increases.