How is the Zakat calculated for previous years on a business that had not previously paid Zakat, especially with the unavailability of financial statements for the merchandise inventory for each hawl (Zakat year) separately? And is it permissible to calculate the Zakat for the current hawl and then pay a similar amount for the previous years?

You have erred by delaying your Zakat for all this time. You must hasten to pay what has become obligatory upon you. If you cannot determine the exact amount with certainty, then investigate and act upon your best judgment. Pay what you believe, most likely, will absolve you, for Allah does not burden a soul beyond its capacity, as Allah Almighty says: "So fear Allah as much as you are able."