Earlier quoted context omitted.
Don't make your readers hold parts of the sentence in their head. Reorder or split sentences until it can be avoided. In other words, use a really small buffer. "You must fill form 331 unless you are over 60 years old, in which case you must fill form 445." "You must fill form 331. If you are over 60 years old, you must fill 445 instead." "If you are over 60 years old, you must fill form 445. Otherwise, you must fill…
I think you gave a good reason why passive voice is preferred in some situations. To me (I am not a native speaker), the default emphasis is on the first words: The police stopped the riot. -> emphasis on the police The riot was stopped by the police. -> emphasis on the riot It coincides with the examples in which passive voice is recommended. > To make something less hostile - 'this bill has not been paid' (passive)…
Rather, there is a rich history in English prescriptivism of misdiagnosing and blaming the passive for all ills.