Shopify theme showing errors in a store. All other themes work well. How can I edit the core file?

I have been working on a Shopify Store built in the Ella theme (from https://halosoft.ticksy.com/). I was working on translator apps and downloaded two of them but since none of them fulfilled my requirement, I deleted them all. Unfortunately after that, I see the following:

  1. The navigation (sub menus) does not appear.
  2. The product image does not change with the variant selection.
  3. The quantity selector on the product page does not work.

All three were initially working. Also, when I use another theme in the same product, all the above points are working. It is not working for any Ella theme. I had taken a backup of the theme as well, but none of the above work for even the backup Ella theme.

Seems like there have been changes in the core file. How can I proceed? Any help will be appreciated.