Oh I wrote it on the strong side, no doubt. Part of the issue here is that the class features have to compensate for the slower spell progression and the very focused spell list, so a direct comparison between the class features of the Spellslinger and a sorcerer or a wizard must by necessity favor the Spellslinger. Wizards have Spell Mastery and Sorcerer's have their metamagic, and the Spellslinger's features are more powerful versions of those features but applied to a weaker spell selection. The damage added features are stronger than their sorcerer or wizard equivalents because the Spellslinger is applying them to weaker spells and fewer of them.

Interactions with Eberron and Ravnica elements that allow new spells added to a class's spell list is a concern I hadn't really put much thought into. Twinning Command and Whispers doesn't immediately leap out at me as broken (sorcerers can do it, after all) but I'll revisit and see if there's a good way to limit this.

I'll probably limit Fire at Will to spells that require an action, as you suggest, at least at 11th level: I might open the feature up again with Arcane Wellspring, as that's around the point where wizards get Spell Mastery.