The first error is because you have deleted the hidden form element named "eCart1_1_ID_Add" from the page. That should be added back to the form and given a value bound to the product ID column in the database.
The second error is because you have renamed your quantity field. It is looking for a field named "ATC_itemQuantity" but your quantity field is actually named "eCart1_1_Quantity_Add". You just have to get those two values in sync by replacing one or the other.