Preserve the selected grid value when a row is double-clicked.

SelectionValue is read-only - it only displays the current selection state. You can't use it to force selections. When a user deselects, Appian clears the selection internally and your conditional logic in selectionValue won't override this.

Use selectionSaveInto to detect the double-click pattern:

selectionSaveInto: {
  a!save(ri!selected, save!value),
  if(
    and(
      length(save!value) = 0,  /* Deselected */
      length(fv!deselectedRows) = 1,  /* Single row deselected */
      fv!deselectedRows[1] = local!lastSelectedRow  /* Same row */
    ),
    /* Trigger double-click action */
    a!save(local!doubleClickedRow, fv!deselectedRows[1]),
    a!save(local!lastSelectedRow, fv!selectedRows[1])
  )
}

There is no double-click in Appian. You could do a "very slow two clicks" thingy, but I am pretty sure this does not meet your users expectations.

As per good design practice number of clicks should be reduced for good UX. I would suggest to have single click selection instead of double (read 2 single) clicks as they are more user friendly. 

Although if I am putting the logic in selectionSaveInto, I am not able to ignore the parameter selectionValue and its by default passing the selected row on single click and in double click, its passing null.