Do you mean what is the alternative to democratic centralism as a method of democracy, or do you mean how, under democratic centralism, does decision making work when the vote doesn’t back the single candidate / option?

EDIT: you might be interested in some of the decision making systems used in anarchist and left libertarian contexts, e.g. occupy movement hand signals and Loomio which came out of the Occupy movement. The Zapatistas movement also has some decision making process that is worth looking into - the way delegates from the movement would return to rural areas to discuss in town halls. Bookchin’s idea of libertarian munincipalism and using limited-sized town halls as a method for communal decision making through discussion and establishing consensus also seems related.

There is an idea that with voluntary, cooperative decision-making, discussion and consensus precedes a vote, so that by the time the vote happens it is merely a formal confirmation of the consensus that was previously formed through discussion. If it gets to a vote and fails, it’s an indication that the cooperative consensus-forming process that should precede the vote did not happen, or something crucial has changed in the time between when consensus was informally established and when the vote was held.

Sometimes yes, or you have to follow it but can slander the decision outside of the party, ie publicly undermine it. Some democratic organs have resolutions on party stance, but don’t make them binding, etc. The strict adherance to party discipline and unity in action is what separates democratic centralism from standard binding forms of democracy, and is more informed by historic experience of what has worked in the past most effectively.