Publishing a non-invalidating comment to a recent paper- should I contact the original authors if I know them?
My comment would not invalidate paper B


So you are in a very different situation than the post you quote.

In your case you can manage so that your comment is not harmfull to the authors of B.

You clearly seem interested into getting a reward by publishing your idea (I do not say this in a negative way), so I would do something like III. I do not think it makes sense to add more authors on an 'opinion' paper.

Do you need some input from the authors of B or are you just trying to be polite? In the last case I would only contact them after having a first draft or at the same time you submit comment. You can turn your email into a 'thanks publisfor this inspiring paper' note. The authors should be happy that you are building upon their idea!