1) Would old-type Reploids with hair bits showing, like Zero, Dynamo, and Red, have actual hair under their helmets?
With lonely_hunter on this one- and I also agree that, regardless of his original design shown in X5, by NOW, Zero probably has a full head of hair and removeable helmet. I remember doing a post about kanekalon some time ago (it's an artificial hair fiber that is purported to be almost identical to human hair), which is what I believe Zero has.
As far as helmet-head... one of the reasons that comes about in humans is due to a combination of sweat moistening the hair, and then pressure being applied to an area that is wet, temporarily deforming the shape of the hair. Kanekalon doesn't have the same properties, and is moisture-resistant, so I'm thinking that Zero prooooobably won't have helmet-hair.
I like to think he has a series of pins to keep his hair back under the helmet, or he's got some kind of gel that he can use to smooth back errant layers so they don't irritate him while he's fully armored.
2: Same deal, and probably it's more widespread among newgens. Most of the Reploids by zero-era have some form of hair, so...
3: Very definitely. The jury is out on whether Lumine's hair is actually soft-hair forced into a rigid shape, or a helmet with hair-like ridges scultped in... Spider, I'm pretty sure, has metal-hair, or a piece of armor that he wears 'like' hair, given those pointy bits.
4: Hmm. Well, given that his hair changes COLOR but not SHAPE in White Axl mode, and given Redips' complete transformation (which works much better if Spider's hair is actually metal/armor shaped)... let's go with no- at least, not until the newgens become polymimetic alloys. *giggle*
5. Because I'm a fangirl, I think Zero looks like Soubi under the helmet in the front, or the way that he's been drawn by Kaneda and others. I REFUSE to acknowledge the bushy eyebrows. XD They'd irritate him while his helmet was on, so he'd probably shave them off. XDDD