Eviolite Kadabra vs. Alakazam defence comparison vs. Rock Slide Machoke (Korrina's Machoke but Rock Tomb is not an option for some reason):
0+ Atk Machoke Rock Slide vs. 0 HP / 0 Def Eviolite Kadabra: 32-38 (51.6 - 61.2%) -- guaranteed 2HKO
0+ Atk Machoke Rock Slide vs. 0 HP / 0 Def Alakazam: 36-43 (51.4 - 61.4%) -- guaranteed 2HKO
(3 level gap, maximum IVs)
The point I was making is that Kadabra can compensate for its inferior defensive bases by holding the Eviolite. Now let's have a look at the special side of the bulk:
0 SpA Emolga Volt Switch vs. 0 HP / 0 SpD Alakazam: 28-34 (34.1 - 41.4%) -- guaranteed 3HKO
0 SpA Emolga Volt Switch vs. 0 HP / 0 SpD Eviolite Kadabra: 25-30 (34.2 - 41%) -- guaranteed 3HKO
(5 level gap, maximum IVs)
As you said, Kadabra will get more out of Eviolite by having better defensive IVs and racking up EVs in def / sp. def. So Kadabra can match Zam in offence while losing slightly in speed and not too significantly in special attack (as Zam can equip items boosting attack without losing to Eviolite Kadabra in defences).
With regards to learning Calm Mind naturally, this happens at L42 and Olympia's lowest-levelled Pokemon is at L44. Abra is in the Slow exp group, so it's very likely that L42 is not reached prior to this battle, and almost guaranteed for any team with more than 4 mons (and I suspect for some 4-mon teams as well). Even should you set up against Olympia (there's a slight chance this will take place), your coverage moves are Charge Beam and Grass Knot. Light Screen from Sigilyph, Air Slash crits while you set up and being Yawned by Slowking all seem like possible ways how to not win here. I think it could be best to just try to take down the first two with SE coverage moves and fight Meowstic and whoever Kadabra/Zam failed to KO with the rest of your team that's able to fight here - seems more efficient than a questionable setup that does require a high level to be possible.
Have a look at the current tier list - Machamp and Machoke are both in C even though their gap in attack is equal to 30. Kadabra doesn't lose to Zam in special attack nearly as much and you could always give it the same offensive boosters as Kazam if you just want a glass cannon. Sure enough, Zam should be just above Kadabra in whichever tier they turn out to be (C or B), but the difference just doesn't warrant a tier difference. You say the 15 base speed difference is so crucial for outspeeding, like, Hawlucha, but given certain levels/IVs/EVs, both will be outsped by Hawlucha anyway, and the insignificant differences in bases may or may not affect in how many hits you die and how many you need to KO back.
tl;dr this is not Steelix, Accelgor or Escavalier - not even GSC Gengar. Same tier (B / C, not sure), with Alakazam just above Kadabra.
Oh, and Scatterbug should be moved from A/B limbo to B/C limbo by now. The accolades it got was before we focused on playing at lower levels w/o Eviolite and since then the criticism of the mon has been pretty much the only kind of evaluation it got, with several nominations for C tier. I don't think it's C at all (though Scatterbug vs. Zangoose or Snorlax is a valid question worth arguing over), but it's not an A-mon even in one's best dreams.
0+ Atk Machoke Rock Slide vs. 0 HP / 0 Def Eviolite Kadabra: 32-38 (51.6 - 61.2%) -- guaranteed 2HKO
0+ Atk Machoke Rock Slide vs. 0 HP / 0 Def Alakazam: 36-43 (51.4 - 61.4%) -- guaranteed 2HKO
(3 level gap, maximum IVs)
The point I was making is that Kadabra can compensate for its inferior defensive bases by holding the Eviolite. Now let's have a look at the special side of the bulk:
0 SpA Emolga Volt Switch vs. 0 HP / 0 SpD Alakazam: 28-34 (34.1 - 41.4%) -- guaranteed 3HKO
0 SpA Emolga Volt Switch vs. 0 HP / 0 SpD Eviolite Kadabra: 25-30 (34.2 - 41%) -- guaranteed 3HKO
(5 level gap, maximum IVs)
As you said, Kadabra will get more out of Eviolite by having better defensive IVs and racking up EVs in def / sp. def. So Kadabra can match Zam in offence while losing slightly in speed and not too significantly in special attack (as Zam can equip items boosting attack without losing to Eviolite Kadabra in defences).
With regards to learning Calm Mind naturally, this happens at L42 and Olympia's lowest-levelled Pokemon is at L44. Abra is in the Slow exp group, so it's very likely that L42 is not reached prior to this battle, and almost guaranteed for any team with more than 4 mons (and I suspect for some 4-mon teams as well). Even should you set up against Olympia (there's a slight chance this will take place), your coverage moves are Charge Beam and Grass Knot. Light Screen from Sigilyph, Air Slash crits while you set up and being Yawned by Slowking all seem like possible ways how to not win here. I think it could be best to just try to take down the first two with SE coverage moves and fight Meowstic and whoever Kadabra/Zam failed to KO with the rest of your team that's able to fight here - seems more efficient than a questionable setup that does require a high level to be possible.
Have a look at the current tier list - Machamp and Machoke are both in C even though their gap in attack is equal to 30. Kadabra doesn't lose to Zam in special attack nearly as much and you could always give it the same offensive boosters as Kazam if you just want a glass cannon. Sure enough, Zam should be just above Kadabra in whichever tier they turn out to be (C or B), but the difference just doesn't warrant a tier difference. You say the 15 base speed difference is so crucial for outspeeding, like, Hawlucha, but given certain levels/IVs/EVs, both will be outsped by Hawlucha anyway, and the insignificant differences in bases may or may not affect in how many hits you die and how many you need to KO back.
tl;dr this is not Steelix, Accelgor or Escavalier - not even GSC Gengar. Same tier (B / C, not sure), with Alakazam just above Kadabra.
Oh, and Scatterbug should be moved from A/B limbo to B/C limbo by now. The accolades it got was before we focused on playing at lower levels w/o Eviolite and since then the criticism of the mon has been pretty much the only kind of evaluation it got, with several nominations for C tier. I don't think it's C at all (though Scatterbug vs. Zangoose or Snorlax is a valid question worth arguing over), but it's not an A-mon even in one's best dreams.