Yes, possessive adjectives in Italian normally require a definite article: il mio capo, la tua casa, i nostri amici.
Exception: singular unmodified family members often drop the article (e.g., mia madre, tuo padre), but this exception doesn’t apply to capo.
• In ufficio means “at/in the office” (location).
• A or al ufficio would imply motion “to the office” (destination).
Since the action takes place at your workplace, you use in.
Yes. Italian word order is flexible. You could say:
In ufficio devo consegnare il documento al mio capo.
This simply emphasizes the location (at the office).