Very sorry for the slow response – have been on vacation, traveling, etc.
You can open a macro enabled XLSM using the Open-Xml-Sdk, but you are limited to directly changing the contents. There is no calculation engine in the Open-Xml-Sdk, so if you change values, it does not change the values in the dependent cells. And custom functions are also not part of the functionality of the Open-Xml-Sdk and Open-Xml-PowerTools.
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