This work studies a series of synthesized Bi1.6Mg0.8-xCuxNb1.6O7-δ (x = 0.2, 0.4) semiconductors and their Li-doped compositions. A detailed structure investigation combining high-resolution neutron-, synchrotron-, and X-ray diffraction methods, as well as DFT calculations, revealed the preferential location of Cu and Li atoms at the Bi sites and Mg atoms at the Nb ones. According to high-temperature X-ray diffraction data, a structural modification caused by the activation of oxygen transport occurs at 200 °C. The linear thermal expansion coefficient was found to be 3.6–4.6·10−6 K−1 (50–400 °C). Magnetic susceptibility measurements allowed us to determine weak antiferromagnetic exchange interactions. The direct band gap was predicted using the DFT-HSE03 hybrid functional calculation, and the optical direct band gap was estimated at 2.3–2.4 eV. Impedance spectroscopy and a dc four-probe technique were also employed to examine the samples' electrical properties. The high mixed electronic-ionic conductivity of the pyrochlores was detected, while the vacancies created by Li-doping in Bi1.5-yLiyMg0.375Cu0.375Nb1.5O7-δ have been found not to affect the conductivity. Besides, the pyrochlores are chemically compatible with the La0.7Sr0.3MnO3 perovskite (up to 800 °C). These make us believe that the studied Mg–Cu- and Mg–Cu–Li-doped bismuth niobate semiconductors can become the basis for composite electrodes to boost their oxygen conductivity.