1) would be a worldpay problem

2) no not unless you would like people to mark goods up from your shop as paid and then you send them out only to find out that worldpay has been bypassed (because that button html code would need to contain your payment response password if I were to code it into the module)

3) implement what ? I think worldpay have killed themselves on this one. The trouble is that the amount of modifications and template chages that people make, which make ZC flexible, means it needs the base href tag of an entirely different engine to run it. I imagine some shop with limited template options or an engine that replaces links in templates, would work but then that's not ZC