No spinner and your whole team is weak to Stealth Rock...? Where is the sense in this? Excavalier is hopelessly outclassed by Scizor, but I'd actually recommend Forretress for Rapid Spin support, so Rocks won't literally ravage your entire team.
Forretress @ Leftovers
Sturdy - Impish
252 Defence, 252 HP, 4 Sp. Def
Forretress is pretty much your only option for a Bug spinner, but he's a good spinner, plus he learns all 3 hazard moves. Since your team isn't stall based at all, you should probably carry Spikes and Stealth Rock over Toxic Spikes. Rapid Spin blows Rocks and Spikes off your side of the field, and Volt Switch helps preserve momentum and lets you switch out of Magnezone.
Venomoth's only real niche is Baton Passing Quiver Dances. You could go with Scizor here, or Yanmega.
Scizor @ Life Orb
Adamant - Technician
252 HP, 248 Defence, 8 Attack
-Bug Bite / Superpower
-Roost / Superpower
This is your typical bulky SD Scizor. Sword's Dance gives you crazy Attack in one turn, and after that he can fire off powerful Technician boosted STAB Bullet Punches. Bug Bite provides a secondary STAB with a bit more power, and Roost heals off Life Orb recoil, however either move could be dropped for Superpower, which slams some of the most common Scizor switchins like Heatran.
Or there's Yanmega
Yanmega @ Life Orb
Modest - Speed Boost
252 Speed, 252 Sp. Atk, 4 HP
-Hidden Power (Ground) / Hidden Power (Fire)
Yanmega actually makes a great late-game sweeper. Lead with Protect, which guarantees you get a Speed Boost, and then sweep. The Hidden Power you choose depends on whether you hate Heatran or Skarmory more. Also, if you do use him, you pretty much have to use a spinner thanks to his 4x Rock weakness.
Illumise actually is a great Prankster abuser, but you're using it wrong. For one Covet is an attacking move and isn't boosted by Prankster. Either way, your team would benefit more from a reliable revengekiller than Prankster support, so you could try your typical Heracross.
Heracross @ Choice Scarf
Jolly - Moxie
252 Speed, 252 Attack, 4 HP
Pretty much gives you flawless coverage on just about everything in the game, unless they make that Levitating Ghost/Steel I've been wanting since Gen 3. Megahorn and Close Combat are great STABs with decent coverage right there, and Stone Edge and Earthquake hit the ghosts and stuff that his STABs don't.