>Did I get that right?
Yeah pretty much I agree with you so far.
>E.g. knowingly not caring about the suffering of others kind of feels bad, not sure if that is a "spook", too.
If you only cared because it was a moral expectation that made you care in the first place, that would be spooked in the sense that you try to correspond to an idea of good that entails that you are only good, a real person, when you care. This idea would be a fixed one and you would behave according to it.
However, I think most people would feel bad for them out of empathy, out of their own desire that others dont suffer.
>What would Stirner say to solidarity?
This is a part that alot of ppl get wrong about Stirner on first glance because of his strong defence of self-interest - but said interest isn't vulgar as in 'only doing things that enrich you materially'. Because Stirner was a pupil of Hegel and not some libertarian market fanatic he sees humans as social. We use each other according to our desires, but that desires also include the desire for friendship or love. As >>1976
quoted, he feels a fellow feeling towards feeling beings, empathy. In fact, it haven't been the egoists (as Stirner defines them, those centering their action on their unique being and desire) who caused and carried out the greatest atrocities in history, but selfless people, involuntarily acting in the name of something outside themselves (god, the race, the nation).
One of my favourite allegories here is that only an egoist gifts a friend a present out of his own desire, while the involuntary actor, who does it because it's the right thing to do, or it's custom and expected from them, always has other motives behind his action.
Egoism doesnt set rules for any behavior, its rather the consequent critique of such rules. If you find it fulfilling to show solidarity because it's your inner desire, it's an egoist act.