Option Explicit
'v2
Sub ChangeGroupItemsCheckBoxesActiveSheet()
    ChangeGroupItemsCheckBoxes ActiveSheet, 0
End Sub

Sub ChangeGroupItemsCheckBoxes(sh As Worksheet, defaultVal As Long)
    Dim shp As Shape, subshp As Shape, ind As Long, iVal As Long
    For Each shp In sh.Shapes
        If shp.Type = msoGroup Then
            ind = 0
            For Each subshp In shp.GroupItems
                If defaultVal = 0 Then
                    ind = ind + 1
                    If ind = 1 Then
                        iVal = sh.CheckBoxes(subshp.Name).Value
                    Else
                        SetCheckBoxValue sh.CheckBoxes(subshp.Name), iVal
                    End If
                Else
                    SetCheckBoxValue sh.CheckBoxes(subshp.Name), iVal
                End If
            Next
        End If
    Next
End Sub

Sub SetCheckBoxValue(cb As CheckBox, iVal As Long)
    If cb.Value <> iVal Then cb.Value = iVal
End Sub

Option Explicit
'v2
Private Sub Workbook_Open()
    Dim sh As Worksheet, cb As CheckBox, cl As Range
    For Each sh In ThisWorkbook.Sheets
        For Each cb In sh.CheckBoxes
            cb.Value = False
        Next
        ChangeGroupItemsCheckBoxes sh, -4146
        For Each cl In sh.UsedRange.Cells
            If HasValidation(cl) Then cl.ClearContents
        Next
    Next
End Sub

Private Function HasValidation(cl As Range) As Boolean
    On Error Resume Next
    HasValidation = (cl.Validation.Formula1 <> "")
End Function