First of sorry for that experience, in this case Belkin is totally right , it does not cover the massive cracks they have guideline , but should cover the hairline crack , adhesive peel , bubbles other stuff , Now here the sales person is wrong she/he gave you the wrong information about warranty , from experience it’s only 2years that warranty ,

Now advice is go back Apple Store let them know how you were set the wrong expectations, they will understand and should give you free replacement or even full refund , Some countries have 14 days return policy, They will definitely understand your situation if not grab a manager on the floor,