Yoghurt making : effects of temperature and duration
First, about the holding time: It is a safety feature. It is meant to ensure that the number of non-culturing organisms that survive is so low that the culturing organisms can overtake them and create a colony of their own, without pathogens. If you reduce it, sometimes nothing will happen, and sometimes you will get a dangerously high growth of pathogenic bacteria. To keep your yogurt safe, you have to do the 30 minutes. 

Second, fermentation time and temperature. When you stay close to the prescribed temperature for a given culture, then you can trust that a higher temperature will give you the same sourness in shorter time, or more sourness in the same time. If you go far away from the prescribed temperature, your fermentation process again becomes unsafe (because some other bacteria are likely to take over before your culture multiplies enough to establish itself). From experience, a deviation of 2C from the optimal is close enough to still make good yogurt, so you can experiment with all three temperatures. What happens if you use 36 C is that you get less tangy yogurt, and at 40 you get more tangy. The length of time you use is pretty much a matter of trial and error, until you get the taste you prefer. 

Note that, for establishing your preferred time, you should use an established culture. That is, you should either always use a new capsule of lab-bought culture, or you should first do a few generations of reinnoculation at constant conditions, and then start experimenting from there. This is because the first few re-innoculations from a new capsule are a kind of "adjusting" fermentations, and your yogurt will be changing its tanginess and consistency even when you keep the conditions (time and temperature) equal.