Catalysis of the reactions: isocitrate + NAD+ = 2-oxoglutarate + CO2 + NADH, and (1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate + NAD+ = 2-oxoadipate + CO2 + NADH + H+.
isocitrate(homoisocitrate):NAD+ oxidoreductase (decarboxylating) activity