Add a dummy association between the datagrid and dataview object, then after you've acquired your list add a relation for each of the lists objects. Now open the form by using the parent object, then constrain your datagrid by using:
dummyAssocationdataviewObjectdatagridObject/dataviewObject/id = '[%CurrentObject%]' (set context of datagrid to private).
Just don't forget to clear the dummy relationship(s) before the next time you use this microflow to open a form.
That isn't possible yet. To the Open Form action in microflow only a dataview object can be applied. Not lists for datagrids. So you have to work around, with for example a boolean attribute on your datagrid object and a constraint on that attribute on your datagrid.