Hitting Cancel button on the DelegateTask page takes you to the ApproveReject page even if there is a custom form.. is there a reason for this? I assume by hitting cancel on the delegate task page, it would take my back to my custom form instead of the ApproveReject page.
I haven't test your case but the usual process of a Cancel button in SharePoint looks for a URL parameter named "Source".
If there is no, it redirects the user to the default page of the list from which is the form.
Is there a such URL parameter in your case? If not, what happens if you add one?