Not sure if this will help of not
Look at the !!!!!!!!! comments in my markup / rewrite
I think you might have over-complicated it
1. I removed the class and just use Workbook events
2. It seems 'fragile', i.e. depending on selection you may get error since Null is returned sometimes in
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Sub CET_CenterAcrossState(control As IRibbonControl, ByRef pressed)
Dim v As Variant
v = (Selection.EntireColumn.HorizontalAlignment = xlCenterAcrossSelection)
If Not IsNull(v) Then ' !!!!!!!!!!!!!!!!!!
pressed = v
Else
pressed = False
End If
End Sub
3. Strictly speaking, subs in your 'EventHandler' module are 'Callbacks', just to be a nit picker 
4. I had to force CET_DisableRibbon to be True to play with it
Sub CET_DisableRibbon(control As IRibbonControl, ByRef enabled)
'Disable ribbon if the Windows username does not match the 9 digit UIN format
enabled = (IsNumeric(Environ("username")) And Len(Environ("username")) = 9)
enabled = True ' Force
End Sub