1. You can maybe create helper entity that consists of attributes from the other two entities that you want to combine in the datagrid.
(example: Entity1=Car + Entity2=Person = HelperEntity CarOwner)
2. Not sure what you really need here, but I’ll write the solution on how I understood. Basically, if you want to click on menu items and show different things, you can use the XPath constraints on your datagrid where the source would be HelperEntity but then you would need to have multiple pages with the same datagrid only different XPath constraints.
Again, not sure what you need for second part but if you explain it more we can help you better :)