Hello,
Since it is bit long time ago when you post this thread, not so sure whether you've found solution already. Nevertheless please try with object type BT126_MAIL, it should be suitable when the email has linked 1Order transaction in CRM.
Regarding using navigate_dynamically, it was recommended not to use the interface IF_CRM_UI_NAVIGATION_SERVICE=>NAVIGATE_DYNAMICALLY as it ignores the concept of outbound plugs. Though it has been used in some of our standard application, for example factsheet.
Regards,
Hongyan