You can do this with Dynamic form Rules.
Make a formula numeric field called [Record ID# mirror] with the formula of
[Record ID#]
It will be used on form rules and for reasons unknown it does not let you base a Form Rule on the [Record ID#] field.
Make a form Rule
When [Record ID# mirror] is equal to (blank)
Action
Make editable [Field A]
Now, if the user goes into Grid Edit they will not be subject to that restriction. So to block that you need to make a dummy form with no fields on it, and then in Forms Usage set that field to be used for Grid Edit for that Role.
