Will T-HUB Work with my shopping cart?
List of supported shopping carts are listed at http://atandra.com/quickbooks-shopping-cart-integration-compatibility.html If your shopping cart is not on the supported list of carts, you can consider custom integration options as per http://www.support.atandra.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=1&nav=0,2

What happens if some transactions fail to import into QuickBooks?
The error message is shown next to the transaction and you can fix the problem and just click a button to import the transaction again.

Is this compatible on Mac?
No this solution is not compatible on Mac

I have multiple websites, will this support multiple sites?
Yes, you will need additional store licenses as per www.atandra.com/buy_thub.html