Re: Colossus
Perhaps I may be of assistance. Yes, an attack does need to do BODY damage to cause Knockback. So attacks that don't do BODY--most NND attacks, for example--don't cause Knockback. Attacks that have the potential to cause BODY damage, like standard EBs, KAs, or punches, can also do Knockback as a result.
Here's the critical bit: a character does not need to suffer BODY damage from an attack to take Knockback from it.
Standard, Resistant, and even Hardened Defenses have no effect on Knockback. The only things that affect Knockback, if I recall correctly, are Density, Growth, and Knockback Resistance.
Let's take look at your example above. Your version of Colossus would, indeed, take 11 STUN and no BODY from the attack. However, Knockback is determined independently of all that. Let's say this attack was a punch thrown by another brick--Arclight, for example. After tallying the STUN and BODY, Arclight's player rolls two additional dice. Let's say she rolls 7. She subtracts this from the BODY total to get 5" of Knockback (12 - 7 = 5). Assuming that Colossus has one level each of Growth and Density Increase, as suggested by some of the other builds, we factor those into the Knockback calculation as well: Growth gives -1" Knockback/Level and Density Increase gives -1"/level. So, in the end, you end up with a total of 3" Knockback that Piotr takes from the attack (5 - 1 - 1 = 3).
In short, you can do Knockback to a character without getting any BODY through their defenses. For example, I remember a fight between several of Colossus' teammates and Juggernaut. Juggy hit Rogue and sent her flying several hundred feet (and through a stone fence, as I recall). Rogue spent the trip shouting "Ow-ow-ow-ow-OW-ow-ow...", but when she got up, she had no bruises, broken bones, or other impairing injuries. In other words, she'd taken a lot of Knockback--enough to send her trough a stone fence--but had taken no BODY damage.
I hope that helps.