I have solved this for Excel, but I don't know how I did it !
Having taken all the actions I described I finally decided to unpin Excel from the taskbar. To my surprise the new Excel icon from MS disappeared leaving the one I was trying to substitute it with - as though there were two icons, one on top of the other.
Now I have twe same issue with Word but am having no success - I think I will post in the Word forum and see what assistance I can get