A) First of all, make sure you had disconnected from the old API integration partner. You cannot use two API integration systems simultaneously.
Contact us at firstname.lastname@example.org let us know that you are switching from your API provider.
C) Jet will put your account on hold for about 2 days. JET will also put seller products to an Unauthorized state so that customer can't make any order.
D) If you add some product with the help of the previous app and-and it is not available on your store then we can't fetch order of that product. In this case, you must generate new SKU for that product on your Shopify store.