Learn Hardcore Korean: toegeun hue samusillo doragaseo seoryureul jechulhaesseo. - After work, I returned to the office and submitted the documents.

후(後) is a Sino-Korean noun meaning “after,” and -에 is the postposition that turns it into a time expression: “after X.”
You can sometimes drop -에 in very casual writing or speech (so 퇴근 후 사무실에 갔어 is still understood), but 퇴근 후에 is more natural when you’re specifying a time.
퇴근하고 uses the verb stem 퇴근하- plus the connective -고 (“and/after doing”), so 퇴근하고 literally “after leaving work” as well. The nuance is tiny:
• 퇴근 후에 treats 퇴근 as a noun (“after work”)
• 퇴근하고 treats 퇴근 as the action of “leaving work.”
Both are fine for “after work.”

Why is 사무실로 used instead of 사무실에? What does the particle -로 indicate here?

-로 marks direction or “to (a place)” when you’re describing movement. 사무실로 돌아가다 literally “go back toward the office.”
사무실에 돌아가다 is also possible; -에 can mark destination. The nuance:
• -로 highlights the direction/path
• -에 simply states the end point or location
Korean speakers often choose -로 with verbs of motion when they want to emphasize “heading to” somewhere.

What does the connective ending -서 do in 돌아가서? Can I replace it with -고?

-서 connects two verbs or clauses, expressing sequence (“do A and then B”) or cause (“because A, B”). In your sentence it shows a sequence: “I returned to the office, then submitted the documents.”
-고 also lists actions in order: 돌아가고 서류를 제출했어 is grammatically correct and means almost the same.
Subtle difference: -서 often feels a bit more causal/sequential, whereas -고 is purely a neutral listing connector.

What’s the nuance difference among 돌아가다, 돌아오다, and 돌아왔다?

돌아가다 means “to return/ go back” from your current position to some prior place (here: from home back to the office).
돌아오다 means “to come back/return” toward the speaker’s current/central place.
돌아왔다 is simply the past-tense form of 돌아오다 (“came back”).
In your example, the speaker is at home when going back to the office, so they use 돌아가다 (“go back”). Had they been at the office speaking afterward, they might say 돌아왔어 (“I’ve come back”).

Why is 서류 marked with 를 in 서류를 제출했어?

를/을 is the direct-object particle. It marks 서류 (“documents/paperwork”) as the thing being acted on by the verb 제출하다 (“to submit”).
Without 를, the sentence would lose the clear object marker and sound incomplete.

Why does the speaker use 제출했어 instead of 제출했어요 or 제출했다?

제출했어 is the informal (반말) past form, casual and used among close friends or in diaries.
제출했어요 is the polite past form (해요체), suitable for most daily conversations with strangers or acquaintances.
제출했다 is the plain/neutral past (해체) often seen in news headlines, written reports, or journal entries.
The speaker chose 제출했어 to keep a casual tone.