Nobody is arguing for a party to have unilateral and unaccountable power. You are deeply confusing how unitary socialist democracy works by alienating the party from the rest of the people, and drawing a hard line between democratic participation in society and the party itself.
Class is not shifting or amorphous. Class is a social relation to ownership of the means of production.
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Capitalists own the means of production individually.
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Workers sell their labor power.
In socialism, workers jointly own the means of production.
You are confusing the type of labor with class itself, which makes it amorphous.
Transition is also not shifting or amorphous. Between capitalism, where there is privatized means of production, and communism, where all production is planned and socially owned, is a period where the means of production need to be advanced enough to accommodate social planning, and administration needs to be advanced enough to facilitate it.
You keep separating the party from the people by erasing their shared relations to ownership of the means of production. The party is of, by, and for the working class, and is accountable to the rest of the working class.