Cassie W. answered 11/22/19
Experienced Humanities Tutor Who Specializes in English and ESL
You have asked some very thoughtful questions. The basic English word order is Subject-Verb-Object. If there is a prepositional phrase, English places at the end or the beginning of the center. We would say "He entered into the office." Or in very formal or poetic speech "Into the office, he entered." We would only put the prepositional phrase of place at the beginning in poetic speech or to emphasize a point, perhaps in a novel. In normal speech, we would almost always say "he entered into the office."
In English, it would be okay to say "Slowly, into the office, he was entering." It does sound more poetic and formal than natural, and it would be unlikely that a native English speaker in America would phrase it this way. As for your last example, "Slowly, into the office, was he entering," it does not sound as natural or correct to English speakers from the United States. In English, we prefer that the Subject comes before the Verb. To have the subject (he) follow (was) and split the (was) from the (entering) is something that is no longer standard in English. You might come across this form in literature or poetry, but it is not something you would want to write in an email or use in a job interview.