Is the alimony decreed by the court permissible despite the absence of consummation, and does legal seclusion (al-khulwa al-shar'iyyah) constitute merely sitting and talking with the husband?

A wife is not entitled to maintenance except through tamkeen (enabling conjugal relations) or a valid khalwa (private seclusion). What you mentioned, merely sitting or speaking, is not considered a valid khalwa according to Sharia. Therefore, you have no right to maintenance, and you must repent and return what you have taken. As for your oath, if you believed the man was truthful in his statement regarding the khalwa, then it falls under laghw al-yameen (a vain or unintentional oath), and there is no sin upon you for it, nor any expiation.