Cool team, Smith ;d I've dabbled with nonconventional HO team such as yours, so I can truly appreciate your endeavor.
I think Magnezone should lose its Choice Specs. Being locked into the wrong move is just too dangerous for this team; I don't think the extra damage from Choice Specs is all that worth it. IMO, your team already has enough fire power to break through most Steel-types on its own, Magnezone's main job is most likely removing Scizor, Skarmory, and Forretress, which does not require much Special Attack to accomplish. To compensate for the loss of firepower you can try Metal Sound over Volt Switch. Unlike Charge Beam, it provides you a much more reliable and quicker way to damage specially-defensive Steels. Metal Sound can even allow Magnezone to have a chance at beating Blissey if it is healthy enough.
Leftovers is the preferred item here to maintain Magnezone's health long enough for it to take advantage of the special drops. However another interesting option is Chople Berry, because it would prevent fast SD Scizor from surprise-killing your Magnezone and then proceed to fuck over the rest of the team. With Chople Berry, Magnezone can survive anything up to +1 LO Superpower from an Adamant Scizor.
As far as the lost slot goes, I'm not a fan of Celebi... its too much of a set-up bait. This team, due to lack of scarfers / strong priority users is prone to being swept by Starmie or QD Volcarona. Tornadus's Tailwind helps, but him alone is insufficient imo. You may want to try Rotom-W instead of Celebi. Rotom-W provides the team with a useful Water, Fire, Ice, and Ground resist. You can invest enough special bulk to survive a +1 Bug Buzz and Thunder Wave Volcarona. Celebi provided some coverage against Conkeldurr, but I believe your team is fast-paced enough to overwhelm Conkeldurr before it can Bulk Up too many times. Rotom-W can Volt Switch out to provide some favorable match-ups, too (ie Tornadus vs Celebi, Terrakion vs Blissey, Terrakion / Magnezone vs Ferrothorn)
Rotom-W @ Leftovers | Levitate
Modest | 252 HP / 140 SpA / 116 SpD
Volt Switch / Hydro Pump / Hidden Power [Grass] / Thunder Wave
Rotom-W comfortably survives a Modest +1 Bug Buzz (69-81%) from Volcarona. Rotom-W's HP Grass has a good chance of 2-shotting Gastrodon, doing a minimum of 50%.
I also second New World Order's suggestion of Sub Dance Terrakion. Substitute not only shields Terrakion from revenge killers, but also eases prediction and protects Terrakion from status. Even with Life Orb, having that 25% Substitute comes in surprisingly handy in dismantling teams, although you would want to use Substitute sparingly (only use it when it really counts).
Thanks for sharing this team, Smith. Have fun!