From my experience:
- Yes with REST you cannot export ID. If you want to SYNC two different applications with same data, then you must actually work with some sort of Unique ID. Without an unique ID if you are exporting the data with export mapping the associations will be maintained, but probably difficult to interpret and arrive and associations.
- See if you can have another service which can send the schema probably. Then your data team, could check the schema. SOAP is also another alternative for data transfer with Schema validation.
- For ODATA, you can export the data with ID. Check for the options here: https://docs.mendix.com/refguide7/odata-representation#associations