Try reading a novel where one charter is "they/them". It's needlessly confusing, and bring the hate, it's a stupid fad.
It's literally been used in the singular for hundreds of years for any individual where the gender is not known, and has never in my life been confusing. For example:
"The suspect entered the store, then they exited through the back."
English is my first and nearly only language and has been for 42 years, and there has never been a time that a singular "they" was not used. It is not a fad, the fad is taking issue with it. And hopefully in 20 years we won't have to deal with this fake "all of a sudden" bullshit, whether it's "they/them," vaccines, or any other nonsense that people suddenly take issue with because some talking head told them to and acted like it was new.
I don't think it adds any more confusion than the pre-existing pronoun confusion you already described as part of the language (your she and her example) and there is already an established answer for it (you don't use a pronoun for one of them, you use their actual name or what you are referring to).
Pretending that it adds some grand new confusion that makes it difficult to keep up with because in very rare circumstances someone who is already really bad at communicating with pronouns (because one would have to have problems with your "she slapped her" reference to have problems with singular they/them) might have difficulty communicating what they mean by "them."