Toot, you're on the right lines. It can obviously be beneficial to produce lactic acid in training as you can't train hard without producing it. However, to restrict breathing is pointless: no one's limiting factor on fitness is their ability to suck air in, and so training this provides no advantage. Maybe if your hermatocrit is over .50 and your blood is thick soup and turns all air into magic power it'd help, but for most people it is not a good way to train. It might make you used to being out of breath, but just training harder with full access to oxygen would be way more beneficial.
Get B3s. B2s on a warm day will be fine but on a cold day you'll be regretting B2s, especially if you are poorly acclimatised. Cold feet aren't fun and Mont Blanc is high enough and can be cold enough that you can do serious damage if your feet. There's a huge difference between the warmth of a pair of Mantas and the warmth provided by something like a Scarpa Phantom or Nepal Evo, and that will likely be the main reason the guide is specifying B3s; it will have little to do with stiffness.
One important factor is that the standard dummy at each test house is different. The test standard gives an idea of dimensions but just as two people who wear men's Medium people are not exactly the same shape, manikins are not exactly the same at each test house.