Cases like these are either solved by giving someone absolute power to steer the project, resulting in squished toes and hurt feelings and potentially becoming

Cases like these are either solved by giving someone absolute power to steer the project, resulting in squished toes and hurt feelings and potentially becoming a HBS postmortem, or accepting that it's going to take a while to reconcile all of the different fiefdoms. There seems to be a lack of translation to "what should actually be built." Management wants a unified API, but my first (and probably incorrect) idea would be to develop an ORM + set of functional libraries which can be implemented independently by each of the involved parties. That has its own risks, but getting everyone speaking the same language is a good place to start.