I managed to fix the non-access to the manifest file. After applying the following, the manifest opens as expected and is fully editable.
Hope this helps. Probably should be added to the 'How to...' guide.
In VS2015 Community, if you go to the following location as if creating a project, you will see a package listed and available for install. It was called 'Universal App Dev tools' or similar. Sorry, I should have taken a screen shot but rushed too much. If not installed, it will be the only option. I installed (long install) and manifest now works as expected.