Echoing the above thoughts that Illusion should absolutely be the DW ability for all three mons. It is an event ability and it's a huge deal flavor-wise. I can't imagine why we would want any other ability.
I think many of you are way over thinking this. Changing a primary ability to a secondary ability is rare in the world of Pokemon; keep things the same. I think Weak Armor is the better ability to keep for all three pre-evos, so maybe variation on No Guard. If I had my way, we'd use the same abilities for all three mons. If that isn't possible, then at least keep the abilities consistent for the two pre-evos. Please don't make these abilities a complicated mess. It looks so ugly and serves little purpose.