The Demo Corps CA has a Minifeat that lets you divide any damage from an enemy attack between everyone in the unit. If I do, does that damage still count as damage from an enemy attack or does it count as damage from the Minifeat?
If I use Malakov2's Feat with the Minifeat, does that mean I can have a Corpsman spread damage to everyone and have most or all of my unit advance for free, per Malakov2's Feat?
No. This rule has gone through a few variations over time, but if I'm remembering correctly the way it works is that only the model directly hit by the attack would get to advance and only if it ended up suffering at least a point of damage. Any shared damage is caused by the bond of brotherhood which isn't an attack. I believe they'd have to suffer at least a point of damage that didn't get bonded to be able to trigger vengeance the next turn.
It's generally better to post rules questions over on privateerpressforums.com where they can issue official rulings; it's the primary use of those forums these days
There have been numerous questions about this on the official forum. Here you can find the latest ruling. In essence, gobber is correct, but you basically have to declare that you "leave" 1 damage on the original model rather than dividing everything and assigning 1 back.
Of course, only the target originally damaged by the attack will get affected by the feat, the rest does suffer damage from the ability and not the attack.