How could the spirits have dealt the adders a Case White level defeat and stayed within clan custom?
Case White’s defeat was made possible by Blakist deep agents and/or ComStar defectors who sabotaged the incoming ComGuard warships. Obviously not consistent with the rules of Clan warfare and trials. And probably impossible for the Spirits anyway, given their isolation and the general Clan disdain for intel.
The defeat of Task Force Vengeance, where the Blakists stopped the Dragoon assault on Mars using Dragau drones, is probably a better model for the Spirits. But that would require the Spirits to funnel their limited resources into some novel orbital defense, rather then protomechs and the like. Maybe dozens of small craft modified with large/fast fusion engines, thick armor/ECM/AMS, Booby Trap self-destruct devices, suicidal crews or basic drone equipment, and little else would do the trick.
At least once. The fundamental problem faced by the Spirits is how vastly outnumbered and outresourced they are. Even if the Spirits cripple the Adder warship fleet and a few Adder galaxies at the cost of some modified small craft, the Adders can just come back with jumpships and more galaxies.