Here is what you can do...
Create a new column in your cart called MinQty and update your add to cart button to store that value.
Then add a calculation to your cart:
TrueQuantity = (([Quantity]>[MinQty])?[Quantity]:[MinQty])
and update the calculation:
TotalPrice = (([Quantity]>[MinQty])?[Quantity]:[MinQty]) * [Price]
Now in your cart display update the value in the textfield form element to show the "TrueQuantity" instead of the "Quantity" and the same thing on the confirm and other pages. Also update your store cart details in database so that the "TrueQuantity" is stored.