Hi, I can probably help you best with this.
Regarding the number of available units, this is not a feature that I believe has been added to the website builder (which is why I have asked the Zyro developers to add it), which means if you want to do this, you’ll have to do it some other way. The easiest I can think of is to do it using a text box, adding the text, and changing it every time the item sells. I know this seems tedious (and I understand why), but until a feature like that is added, it’s your easiest alternative.
Secondly, regarding the subcategories, I believe its meant to reset all the fields and make you choose the template/copy the content again (not sure why, though). I can request that this be changed or an auto populate/copy template feature be added. In the mean time, I apologize for the inconvenience.