There's no point cutting out plaster etc. If you remove the moisture the mushrooms can't grow. Use a fungicide to kill off what's there (or bleach as I said earlier) and keep the area dry.
I think if anyone should be spraying under the floors, then it should be your upstairs neighbour. They can do this obviously from above & without chopping holes in your ceiling.
the plaster may have been damaged, especially if it is a plasterboard ceiling. I suggest you notify your house insurers.
I had a slight long-term drip making a ceiling wet patch, during redecoration i poked it with a scraper and a section was rotten and broke away. Difficult to fix if you are not a plasterer.
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