The necromorphs being able to instaneously transform is a bit silly and unrealistic, but since the first one did it, the second had to as well or else go with different enemies altogether. So call it a flaw of the series, but DS2 had to have it and in fact DID slow it down a bit by including a transformation sequence.
As for horror, I think it works despite being genre fusions as you consider it. All the actual horror comes from the atmosphere and every part of the game where the enemies are absent anyway. The enemies are not the horror.
As for the helmet: depends on the suit. On the construction suit, I do think it's a bit unnecessary but see the next reason and keep in mind that they can't have helmets doing too many different animations. On the riot/security suits, think "cop that may or may not need to visit a vacuum." That helmet is designed to automatically create a perfect vacuum seal and only needs to withstand melee damage, and needs to be removeable by a cop carrying riot gear. As for the military suit, think about it, a lot of military gear is designed with armor but its priority is wearer mobility, therefore sacrificing some armor. The "military" Advanced Suit is a combat-ready space suit that does have extra bulk for armor but primarily serves as the above helmet. Being bulletproof isn't the primary concern, being impact-resistant is, which is aided by the articulation points having some give. Also, how is being bulletproof by virtue of armor plate going to protect you from laser weapons, which the military clearly has access to? As for putting undue weight on the neck, both the front AND the back(yes it does split in half, not just going down one side) halves of the helmet store on the TORSO. The chest compartment and the back compartment.
There you go. I may have nitpicked a little, but so did you. But at least I looked beneath the surface.