You can substitute this card for any 1 Fusion Material Monster, but the other Fusion Material Monster(s) must be correct. You can Tribute Fusion Material Monsters on the field, including this face-up card; Special Summon 1 corresponding DARK Fusion Monster from your Extra Deck.
No, they (at least the more recent ones like Nerokius) pretty clearly say they first have to be Summoned with the listed method. (And since it's not a Fusion Summon, you can't use them as regular Fusion Subs for these Summons, either.)
So basically hex sealed fusions' second ability is pretty useless unless you are summoning red eyes slash (I heard). All hero monsters and a number of strong fusion monsters cannot be special summoned. I hate you game.
The Dark - Hex-Sealed Fusion / The Light - Hex-Sealed Fusion / The Earth - Hex-Sealed Fusion: This monster's second effect cannot be used to Special Summon Fusion Monsters that can only be Summoned by Fusion Summon, such as "Dark Paladin".
Doesn't work for Dark Paladin. Maybe B. Skull Dragon, but that still requires you to have either REBD or Summoned Skull on the field. Otherwise, you might as well be using any other fusion material substitute.