that's a complicated question
Linuxboy is the right guy to answer that question, but he's on hiatus so I'll do my best
You are absolutely correct, the temp ranges for incubation favor ST. That's the point. ST can be tricky to get started, if left to spoil for example ST usually won't be what curdles milk.
Our goal is to target the native ST, and get it jump started and to develop a strain that is well adapted to the processes of cheesemaking.
What we want is to get ST for early acidification, so that our cheeses will reach their pH in the right time frame. Fortunately for an alpine make, acidification rates really aren't that important. We are going to remain above 6 the whole time during the make.
It was explained to me like this,
Lactobacilli are the most important flavoring element. IF you use pasteurized milk, you need to add a good deal of them. So tha's why when making gruyere, etc. you need to add a culture with a good LB strain in it (it actually doesn't matter which LB strain you use. Everybody think Helveticus is the right strain for Alpines, but guess what in Switzerland they use Delbrueckii. They all behave similarly, close enough for our purposes, and produce the same byproducts, ie. flavor.)
BUT if you use raw milk, it is full of natural LB. In fact, Pav told me once you really don't even need to add any at all. I like to though, because I want to get as much LB flavor in there as possible. The Alp where I learned, we added yogurt to get stronger LB in the culture, and the flavor was more intensely nutty and spicier than other Alpkäse from nearby Alps, that's what I want.
But the thing we have to remember with bacteria growth and flavor is, if it is strong enough to beat out other bacteria it really doesnt matter how much you add. Bacterial rates at startup almost don't matter, all that matters here is that your bacteria are numerous and active enough to beat out any other competitors. That's why we add a starter instead of just letting the milk ripen for a couple of hours by itself. The rate of reproduction past the first few minutes depends almost none on how numerous they were to start (in an ideal environment, bacteria doubles its population every so many minutes, depending on strain) what matters more are conditions (the doubling rate is quicker at ideal temp ranges) food supply (again, they reproduce faster witth more food) and competition (competitors take away food, and may attack)
Remember a bacteria reproduces by dividing themselves, so that's why the population doubles.
So if you are following me here, timewise we are behind a very small amount if we started with half or even 1/4 as many bacteria in our cheese, because the cycle is pretty quick.
SO I guess the simple answer to your question is,
If you are using raw milk and passing down a whey culture, yu don't need to add LB from any outside source. But you still can, just keep in mind that the added LB will make your cheese sharper quicker (also, drier quicker, and potentially sour) Alpkäse is actually quite sour because of the strong LB growth, but you can hardly detect the sour over the salt and spiciness.
If you use processed milk, you need to add LB from some source. You can use yogurt, that's what we did on the Alp. That's the same reason why I cultured a second batch at a higher temperature.
Despite the high temps used in treating the milk, ST and LB both should have survived from your milk. They are both very heat tolerant bacteria with thick cell walls and good natural defense abilities. These two bacteria are the reason Alpine cheese works, where part of the process is to cook the curd up to a point where pathogens die, but these two workhorses survive.
So our culture actually has LB in it, but by incubating it just below LB's favored range, we ensure ST is stronger.
ST will grow at 45C. I make yogurt at that range some times. But LB will just grow a little faster.
The logic for incubating at 42 is so that ST gets stronger. We are really trying to wake up the ST to get our early acidification (that is, the first two days of the cheese's life) the LB has months to get going, we don't need to worry about it too much.