"Most of that is still describing the "what". Only the "(mutations, if you like)" describes the "how". And one might still wish to know how the non-photosensitve cell chose to become photosensitive"

That's the point: no choice. It just happened. But having happened, it gave its possessor an advantage. It was photosensitive. It wasn't sensitive to magnetic or electric fields, to temperature, to the chemicals we feel as taste or smell, to pressure.... other cells (some, sadly, not in humans) did mutate in that way, and conferred the associated advantages on their owners.

You could sit down and list all the changes that didn't happen, perhaps because they are impossible, perhaps because they are possible but the chance mutation hasn't happened yet, or perhaps because it did happen, but its one and only carrier was just stepped on by me as I walked along the path outside.

I'll reiterate an important point: humans (or any other organism) didn't HAVE to be the outcome to date. They just were. If it were possible to rerun evolution from some earlier stage, it's perfectly possible that cogent organisms might never have developed. Or they might have been based on a different body plan (see Gould's Wonderful Life for some of the early body plans that did exist). In that case, they would have been talking about, say, crustaceans rather than mammals as the pinnacle of creation.

I'm certainly not saying that there are no mysteries- the greatest of which is why we (think we??) feel anything, why it seems to be ME that things are happening to. For my own part, I have no evidence about that; I just make the temporary assumption that feeling is a property of all sufficiently complex systems, a property of arranged matter.