What If: Remove Kleptomancy and move Summon Aery over to the Inspiration tree in its place.

At this point, Klepto just seems like it's going to be more trouble than it's worth to try and balance. Once you "fix" it on its abuse cases, it's going to be mediocre to horrible on everyone else.

Why not just remove Kleptomancy and put Summon Aery in its place on the Inspiration tree instead? Aery and Arcane Comet being on the Sorcery tree together honestly feels a bit redundant, even with the former's added utility, and the quirkiness of having a pet that aids you in battle seems like an Inspiration tree kind of thing anyway.

To maintain the lack of damage on the Inspiration tree, change Aery's damage component to a sort of looter effect.

Summon Aery

Damaging an enemy champion signals Aery to dash to them, whereupon she lingers for up to 2 seconds. The first time Aery's target gains gold from an ability or item effect (except for another Summon Aery), or from killing a lane minion, you also gain gold equal to 2 + 10% of the gold the target gained (this effect doesn't steal gold from the target, just grants you bonus gold based on what they gained).

Healing, shielding or buffing an allied champion signals Aery to dash to them, shielding them for 30 - 80 (based on Level) (+25% AP) (+40% Bonus AD) for 2 seconds.

Aery lingers on the target for 2 second before flying back to you, and cannot be sent out again until she returns to you. Aery is initially very slow, but gradually accelerates, and can be picked up by moving near her.

As for replacing Aery's space on the Sorcery tree, perhaps something that's part Thunderlord's and part DFT?

Deathfire Ignition

Dealing damage to an enemy champion with an attack or ability stacks Ignition to the target for 6 seconds, up to eight stacks. Single target attacks or abilities apply two stack, while area-of-effect or damage over time abilities apply a single stack per instance of damage.

At eight stacks, Ignition detonates, dealing 45 - 150 (+40% Bonus AD) (+30% AP) adaptive damage to the target.

Enemy champions recently damaged by Deathfire Ignition cannot gain Ignition stacks again for a short time.