Thanks for asking these questions.
1) An easy way to set all products to have the same google_product_category would be as follows
if ProductID >= 1 , then google_product_category= Apparel & Accessories > Clothing > Tops > Sweaters
2) Exclude from Feed = True. (good guess. "1" and "yes" also work)
3) Variant description isn't something we feed, except as a substitute for the product description. Please can you share more information about the Google warning?
Vortx / AspDotNetStorefront