I'm echoing DetroitLolcat's thoughts about changing the abilities; just two pseudo-legendary keeping its pre-evo's abilities: Garchomp and Metagross.
However, one must think about the Bug factor here and look at three-stage Bugs: they all change abilities. In order to keep them logical flavor-wise I think it should be ideal to change the abilities but keep them at three.
Shield Dust seems legit as Wonder Skin does.
Cursed Body doesn't seem too bad but I find it hard to fit into these bugs because it's an ability that always comes with Ghost-typing. Likewise Light Metal/Heavy Metal comes with Steel-typing.
Considering DHR's post about how there are logical transition of abilities, Shell Armor seems completely legit for Weak Armor. No Guard and Illusion are completely difficult to translate, though.
Shield Dust sounds nice to go with Illusion (at least in my head), even Wonder Skin but I'm not sold about Compoundeyes into No Guard; they just don't seem to relate at all.