So glad your cat is feeling better. Abcessess are horrible to deal with but if the puss is all out now it should be a lot better and less painful for her. As your vet says the main thing is to keep it clean until it heals.
Is she micro-chipped? You can get a catflap that works with the microchip which might be more effective than a magnetic one. They are quite expensive though.
We had a bully cat near us when we first moved here 10 years ago and he made my cat's life miserable. He was a big ginger tom and he used to spray all around outside even though our boy was neutered. If Josh-cat went out ginger tom would be there hissing and growling at him. I threw all sorts of things at him (an empty champagne bottle once!) and turned the hosepipe on him too.
It seems as if your house has become a local 'free meal' place for the cats in your area. Word has spread and they all want to come
I can't think of anything that might deter them. I assume your cat is spayed? Other than that I can't see why you get the intruders.

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