The hidden field was there, it just wasn't inside the form with the other eCart form elements (best guess). That error went away, but still receiving the 'ATC_itemQuantity' error.
I changed the name of the quantity field, but the error remained. Noticed line 144 was still named "eCart1_1_Quantity_Add" so I changed it to "ATC_itemQuantity_Add" and also to "ATC_itemQuantity" but still received the same Undefined Index error.
Also noticed the quantity field is now named "ATC_itemQuantity" and the Add to Cart button is named "eCart1_1_Quantity"; don't know if this is a problem.