It has been two weeks since I reverted to my original site
After reverting to your original site, what happens if you request one of new site URLs? Do they still return the content, do they redirect or do they return 404?
Although you have reverted, you have in the meantime flooded Google with new URLs from your new site. Google will need the time to digest this + the new URLs should not be returning the content, they shoud be either returning 404 or redirecting back to the original URLs, providing that you will not proceed with the upgraded site for a some time.
But if you reverted now and want to put a new site up couple of months down the track, then this is probably not a good idea as Google will be confused. In which case, I would rather revert back to a new site, would NOT redirect these important URL to the home page, instead I would redirect them to the new URL that has exactly the same
content as the pages that ranked (including the same page title etc) and fix any other technical issues and then wait.
And lastly, you did not say whether the upgraded site has a different information arhitecture and different navigation paths as these can also be the reasons why the ranking of your upgraded site dropped.
It is much easier to do website redevelopment that includes IA and URL changes if the site is ranking poorly since with the new IA and carefully consolidated redirects and with no technical issues, the usual path is only up.
But if the site is already ranking well then the changes should be done in stages with enough time between the stages to allow Google to digest the changes.
So if your URLs had to change because of the new cart, then put the new cart with new URLs but try to keep the site structure and how the pages interlink initially the same and redirect old URLs to new URLs on the one-to-one basis.