How do you ensure that no-one tries to occupy the seat next to you? Do the cabin crew indicate the seat in some way, or is it left to the passenger to tell his fellow travellers that the seat is reserved?
A bit of both, but it's no problem. Placing your hand luggage on it will deter all but the most persistent and the crew will help them find another seat, if they need one.
What happens if you board the plane and can't find two seats together? Do the cabin crew make someone move to make space for you? What if the only empty seats are alongside families or couples? Or speedy boarders who have paid extra to get the seat they want?
The crew will find you two seats together.
It is not a recipe for disaster, it works fine.