Hi @Mel Calucin
Sorry for this delay. The excel file in SharepPoint Online is related to Office Online (not Excel 2016), so setting the registry setting DisableMergeInstance in HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options could not work. I could only find an article to tell how to force Excel to open in a new instance by default on client instead of online.
Have you tried to open the excel file in SharePoint in the client application through library settings > Advanced settings:
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