I have a form called by entity A, containing a dataGrid over association A_B. When saving, I need to commit both A and B but here is the thing - I cannot add a parameter B to the microflow (I get an error that "there should be one parameter of A"). I cannot access the datagrid over the reference $A/ - it only shows attributes of A.
If you have a data grid in A containing B, the B entity records are already saved when creating them, so there seems to be no need to commit them separately.
Microflows can contain multiple input parameters but not from a button in a form directly, like on your form A. However if needed you could retrieve the records in B from the input parameter A and then commit the list of B entries that you retrieved.