1.)Nope, the helmet shown in the video are replacing the WW2 M1s, not the patkas. Patkas have their advantages, but they do not protect the top of the head (Falling debris!), the lower back parts of your head and your ears !
Again, protection from direct bullet hits is not the main purpose of modern helmets. The chance of getting injured/killed by debris, splinters, trauma etc is much higher than receiving a direct bullet hit into your upper parts of your skull.
2.) Not saying that the build quality hasnt improved at all... it has! But its still sub standard. But the main problems are still its ergonomics, weight etc
Furthermore, most IA troops dont have any optics
3.) Again, good knee pads dont restrict movements nor do they add any significant weight.... Outdated helmets, BPJs, overweight rifles etc etc etc are far far far more concerning than knee pads, which weigh nothing compared to the stuff mentioned above.
FAST helmets are normally issued to SF units only. And the gap is meant for communication equipment.