"2)Why on high profile, it's recommended to increase speed?"
L/D (better gliding performance) is almost always slower than descent speed. Speeding up generates more drag which results in your energy being closer to the profile.
Also increasing speed results in a faster rate of descent getting you closer to your profile.
The reserve applies if you get low on profile. Slow up closer to L/D speed. You should get closer the VNAV descent profile as your gliding performance at L/D is shallower than it is using the VNAV descent speed..
Adding power isn't the most efficient way to correct a small deviation below profile.