How does charge conservation apply in redox reactions?
It applies absolutely.........
Mass and charge are conserved in all chemical reactions, and for redox reactions this conservation of charge is quite explicit. Redox reactions invoke electrons as virtual particles that are LOST upon oxidation, and are gained upon reduction.
See this old answer for a few examples.
For the use of electrons to balance mass and charge, consider the oxidation of thiosulfate anion,
Are charge and mass balanced here? If no, then you know it's not right. Of course, this reaction would be accompanied by a corresponding reduction.
For thiosulfate, I have always liked to consider the central sulfur as