In this case the thread was duplicated in the support system and in the forums.
I will additionally summarize the support communication for further reference here so that more people can have access to the answer:
Our general recommendation for cases with complex relationships between persistent and non-persistent fields is to handle the data binding manually without using the Auto Update capabilities of the grid. You can use the Add/Update/Delete events of the grid to plug you code in for that purpose.
the Telerik team