suplimentary advice from an HP ?

No, all points are not added together. Daily living and mobility are separate and because of this those points wouldn't have made any difference, there still wouldn't have been any award.

For those mobility points, it seems like the decision maker didn't agree.

Do remember that PIP isn't awarded based on a diagnosis, it's how those conditions affect your ability to carry out daily activities based on the PIP descriptors.

For your submission for the Tribunal you should state where you think you should have scored those points and your reasons why and add a couple of real life examples of what happened the last time you attempted that activity for each descriptor that applies to you.

I will also add that having some understanding of what the descriptors mean also helps. As you thought the mobility points were added to the daily living points then having a read of this link will definitely help. You'll have plenty of time to read it because waiting times for hearings are huge across most of the country.