In that you are in opposition to the entire biology establishment, who say that wolves and dogs *are* two different species. What do you know that they don't...? ;-)
Interbreeding used to be one of the tests for species difference. There are so many cases where this clearly isn't workable though (lion/tiger and horse/donkey, for example) that this was always open to dispute. DNA sequencing opened this up so that speciation can now be defined by genetic differences, regardless of physical similarity or ability to interbreed.
Only the "(mutations, if you like)" describes the "how".
Yep, and that's it. They happen so often that you often don't think about it - some potentially-advantageous human mutations are: being physically stronger than everyone else; being significantly taller or shorter; reaching puberty earlier or later; wider hips for child-bearing; predisposition to put on fat; etc. If you're a caveman and a random shuffle of the genetic cards pops up one of those, you're more likely to reproduce, and your offspring are then also more likely to have the same characteristics.
You say that the odds are astronomical. Well, I'll bet that if you picked maybe 20-30 friends of yours at random, some would be noticeably more muscular than others, some would be significantly taller or shorter than others, and some women would have significantly wider or narrower hips than others. "But this is normal", I hear you say. And so it is - but it's how this evolution thing works.
In pre-medical times, your thin-hipped women friends would almost certainly die in childbirth (and so would the baby), whereas large-hipped women could crank them out without problems. If the default is thin hips and most thin-hipped women die in childbirth, it only takes a random shuffle towards slightly wider hips and that woman and her offspring will be more likely to survive. Repeat the random shuffling, and wider hips will give more security in childbirth. You may care to observe that narrow hips and large birth size of babies are most common in Western societies where medical knowledge has prevented hip size being such a significant issue.
And one might still wish to know how the non-photosensitve cell chose to become photosensitive.
See, you're still using the word "chose". That's bogus - there's no choice. Did you choose your eye, hair and skin colour? Or did your hair follicle cells say "I'm going to decide to produce blonde/black/brown hair"? Or taking more extreme examples, do people choose to be blind or deaf from birth, or have sickle-cell anaemia? Of course not! So why do you call this a "choice"? It's random luck - sometimes the luck is good (if the random shuffle gets you a 250lb Arnie musculature) and sometimes the luck is bad (if the random shuffle gets you blind/deaf/mute).
Photosensitivity is a characteristic that many cells have - the issue is the extent to which they have it. Random chance will shuffle the amount of photosensitivity, and the prehistoric worms that roll a 12 get to detect a predator and dig into the earth to escape first. The prehistoric worms that roll a 2 can't detect the predator and get eaten.