Trace is nice because it slows a few Pokes that would have an easier time coming in on CAP4, thus widening the idea of "no one true counter, but many checks". Heatran and Thunderus-T have their common STABs made null and void, while Tracing Tornadus-T's Regenerator would heal off Stealth Rock damage when we want to get out of there. It also fits thematically with Illusion, in that it shows adaptability and is also situationally useful.
Multiscale could be cool, but almost constantly would be chosen over the other two abilities. You'd have more reason to keep Stealth Rocks off the field than Illusion, especially if we're not planning reliable recovery. And an intact Multiscale almost always guarantees you a boost of some kind. It would deter us from slotting boosts like Quiver Dance and Agility in CAP4s movepool. You benefit more in positive situations with Multiscale than you would both Illusion and Weak Armour, making these abilities only rarely useful. Sure, you could still benefit from a -1 Def, +1 Spd on a smart switch, or still bluff an Illusion if you are disguised as an appropriate Pokemon, but both are hardly as useful as being able to come in on almost any attack and survive.
Though, I do see some reasons to use it as well. If it really was a one off Multiscale, having CAP4 have no recovery other than Rest, I could see places where it'd be more risky to run it over Illusion or Weak Armour, i.e. coming in on a check/counter while Scarfed, surviving the hit, and getting the jump. It's both risky because its one-off, and both STABs are easily resisted, making prediction harder. Wish support could let you try again, but is also risky.
I dunno if it works this way, but if we made Multiscale the tertiary ability, have CAP4 be Male Only in Dream World, and limit boosting moves to egg moves, we'd have good reason to choose Weak Armour and Illusion over it. This might be encroaching on moveset discussion, and if so I'll drop it and discuss Multiscale on its own.