We present a comprehensive high-level quantum-chemical investigation of the electronic structure, potential energy surfaces (PESs), and rovibrational spectroscopy of the MgC2 system, motivated by the recent astronomical detection of cyclic MgC2 in the carbon-rich circumstellar envelope IRC+10216. Three stable isomers are characterized: the known cyclic global minimum c-MgC2, the linear isomer l-MgCC, and a previously unreported centrosymmetric linear structure, l-CMgC. The latter exhibits an unusual near-degeneracy between its lowest singlet and triplet electronic states, revealing a delicate balance between electron pairing and spin polarization that reflects its pronounced multiconfigurational character. While c-MgC2 is accurately described by a single-reference wavefunction, the linear isomers require a multireference treatment and display strong anharmonicity, particularly in their bending vibrations. Three-dimensional PESs were constructed at the internally contracted MRCI+Q/aug-cc-pV(5+d)Z level and employed in perturbative and variational rovibrational calculations, yielding accurate rotational and vibrational constants together with rovibrational energy levels extending to approximately 2000 cm^{-1} above the zero-point energy. The calculations reveal pronounced anharmonic resonances in c-MgC2 and highly anharmonic bending dynamics in the linear species. Spectroscopic parameters are also reported for the ^{25}Mg and ^{26}Mg isotopologues. Two MgC2 protonated species have also been identified and their accurate structural, rotational and vibrational characterization provided. Overall, the spectroscopic datasets obtained in this work offer reliable predictions for future laboratory investigations and radioastronomical searches for MgC2 isomers, isotopologues, and protonated derivatives, thereby extending the molecular inventory of magnesium-bearing species in astrophysical environments.