vel'koz all for one

Yes. So does LeBlanc's Sigil, Vayne's Silver Bolts, and Gnar's 3-hit. I don't know about the others but i'd assume they do too

Yes.

Lets say Vel'koz #1 lands 2 spells, and applies 2 marks. Vel'koz #2 can throw out his own spell and proc the marks.

Same applies to any champion with a "hit 3 times for bonus" mechanic.

One for All Vel'Koz is fun. The first One for All game I played was my whole team as Vel'Koz against a whole team of Nunu.

And yes, the passive stacks