Why is 本人 inappropriate in an objective text?
本人 means 我, I in the formal circumstances, which is used to make a serious statement/declaration, such as in an allegation. For example, 本人郑重声明: 此事与本人没有任何关系！If you use this sense of 本人 in non formal settings, that would sound either pretentious or a bit joking. For example, 本人今天要去买东西 sounds pretentiously funny because 买东西 is really a very casual thing and people don't expect you to mess it up with such a formal word 本人, which add some taste of funny.
In casual speech, 本人 means in person, oneself. See these examples:

这照片是你本人吗？// are you the very person who is in the picture? Or is the person in the picture yourself?
你是Ludi本人吗？// are you Ludi himself?
你本人能(亲自)来吗？// could you come in person?

吾國 and 該日 are old fashioned and we don't usually use these words in modern Chinese, especially within the mainland China.