What I ended up doing to fix this was I added all the DLLs referenced by MonoGame.Extended.Content.Pipeline.dll to its folder, as defined in the Content.mgcb config file at the line below:
As a permanent fix to get going with my project and not deal with these needless issues
1) I'll remove ALL nuget references in my project
2) I'll add the actual referenced DLLs to a library folder in my project and will move references in my VS solution and Content.mgcb config file to these DLLs
3) I'll update the referenced DLLs when necessary
Hope that helps anyone else dealing with this.