What is the ruling on taking one's time in making the decision to wear the niqab and not rushing into it, as long as the questioner has been unable to ascertain whether it is obligatory or merely recommended? And can she adopt the easier opinion and postpone wearing it until after marriage if she remains undecided about the ruling?

It is not for the general populace—those not specialized in Islamic sciences—to delve into matters of scholarly disagreement to weigh between them. Rather, this is the domain of mujtahids (qualified jurists) and expert scholars. An ordinary person or a student of knowledge who is not well-versed should seek fatwa (religious edict) from someone whose piety and knowledge they trust, and act upon their قول (statement/opinion). Among the commendable principles is "exiting disagreement," so if it is possible to act upon a universally agreed-upon opinion, that is preferable. Scholars are in agreement that wearing the niqab is a فضيلة (virtue) and a سنة (Sunnah), and they differed on the sinfulness of one who does not wear it.