Option 1)

A full clean uninstall of Visual Studio 2015 did the trick to get SharePoint Designer 2013 working again for me. All Workflows finally open OK again!!!!

Here are some highlights of lessons learned, that I hope can be used in case anyone may encounter this in the future:

1) Always install SharePoint Designer 2013 before Visual Studio 2015.

2) If Visual Studio 2015 IS installed before SharePoint Designer 2013 then you may encounter an error :

a. “Server Side Activities have been updated”

i. https://answers.microsoft.com/en-us/msoffice/forum/all/sharepoint-2013-designer-workflow-issue-server/f36da602-5000-43ed-97c4-a273a5600c15

3) When uninstalling Visual Studio 2015, the default uninstaller does not “clean” everything. The below tool can be used to completely remove Visual Studio 2015 to ensure that any new install starts with as fresh clean environment:

a. http://ridilabs.net/post/2017/03/11/Clean-Uninstall-Visual-Studio-2015-to-Install-Visual-Studio-2017.aspx#.WqY4wleJIuU

Option 2):

Please close your SharePoint Designer application, clear/delete the cached files and folders under the following directories from your server installed SharePoint Designer, then check results again.

<user profile>\appdata\roaming\microsoft\SharePoint Designer\ProxyAssemblyCache
<user profile>\appdata\local\microsoft\websitecache\<sitename>

Option 3)

Install Patch:

https://support.microsoft.com/en-us/help/3114337/january-12-2016-update-for-sharepoint-designer-2013-kb3114337