In most semi-hard cheeses, aren't we looking at a 0.1 drop from the starting pH,
When using DVI, yes. This is to try and mirror traditional cheesemaking as much as possible. DVI changes the dynamics of the acidity vs bulk cultures.
in alpine cheeses, the rennet added before the 0.1 drop, so coagulation takes longer, and v/v for a cheese that ripens longer?
yes, this. Though in alpine styles, it is not so much rennet that helps with maturation (it's denatured), it's plasmin. We try for a longer time to floc/higher rennet in alpines to achieve correct buffering kinetics. Or to put it another way, to retain a lot of calcium.
How does the amount of rennet change the bell curve for maturation?
Works in combination with the set time, culture selection, and affinage. More rennet generally means a bit more flavor development potential and breakdown, but lower usable optimal life to sell. We're talking very fine tweaks, though. It's like saying, I want a 4 month window of optimum flavor coupled with a 4 month affinage vs 6 month affinage and 5 month window. That's often how rennet amts are tweaked on the floor, also taking milk specifics into account in terms of composition.