I note that when attacking a horde, a space marine inflicts 1 magnitude per 2 degrees of success. If a space marine swift attacks a horde, do they effectively inflict 2 magnitude per 2 degrees?

So, if a weapon skill 50 marine with lightning attack and whirlwind attack gets 4 DoS vs a horde, he is reducing magnitude by 10 (lightning attack - 4, base magnitude loss vs horde - 2, whirlwind attack - 3)?

With Swift Attack/Lightning Attack you're landing multiple hits. So I suspect it may even multiply up - I can't recall if Whirlwind of death is 'per attack' (like storm of iron) or 'per hit' (like blast weapons).