We've just recently bought and started using Orbeon.
When the user edits a form, and want to save it, he press the save button, and we defined it to send the Orbeon-style XML with all the controls values to my C# REST API. When the user wants to continue editing the form, I call the Orbeon form, and there is a trigger, what fires when page loads, and it starts a REST API call to my C# REST API, to get the control values. Now we have to define map EVERY value. Eg. ControlSurname -> ControlSurname, ControlLastname -> ControlLastname ... etc.
Is there a way, that my C# API gives back the same Orbeon-style XML (maybe some values changed due to the business logic, but not the structure), and Orbeon parses it and put all the control values to its place? (Without mapping all the 87 control?) (It works in the save methon, you dont give a mapping when save...)
You can configure Orbeon Forms to load the initial data from a service. See the section Initial data from service for more on this.
The benefit of this approach is that you won't have to do any the mapping work you were referring to, and the drawback is that you have to make sure that the structure returned by your service corresponds to what the form expects, or your form might end up with missing fields or other problems.