本人 and 他人: the relevant person and other people
Read the applicant, the person asked about a rumor, someone else's letter and the wish to help others. 本人 need not be the speaker, and 他人 does not always identify a completely unknown person.
The person relevant to a form or a rumor
「この申請書は本人が記入してください。」 (The applicant must fill out this form personally.)
Another example reports asking the relevant person directly whether a rumor was true. 本人 refers to the person concerned in each scene; asking about the rumor does not itself confirm it.
Someone else's letter and a wish to help
「他人の手紙を勝手に読んではいけない。」 (You must not read someone else's letter without permission.)
Another sentence expresses wanting to offer help even to strangers in difficulty. Keep the prohibition and the wish distinct from actual reading of a letter or completed help.
Identify the person in that situation
本人 points to the person concerned. In the form example, that person is the applicant, who need not be the person speaking the instruction.
他人 can refer to another person. Someone else's letter need not belong to a person completely unknown to the speaker; identify whose letter or situation is being discussed from the context.
Must 本人 in the instruction about filling out the form be the person speaking that instruction?
Show the answer and why
No. It refers to the applicant concerned. The instruction does not report a completed form. Likewise, asking the relevant person about a rumor reports the question, not confirmation that the rumor is true.