GHZ2/GLASS-z12, one of the most distant galaxies found in JWST observations, has been recently observed with both the NIRSpec and MIRI spectrographs, establishing a spectroscopic redshift z _spec = 12.34 and making it the first system at z > 10 with complete spectroscopic coverage from rest-frame UV to optical wavelengths. This galaxy is identified as a strong C iv _λ _1549 emitter (EW = 46 Å) with many other detected emission lines, such as N iv ] _λ _1488 , He ii _λ _1640 , O iii ] _λ _λ _1661,1666 , N iii ] _λ _1750 , C iii ] _λ _λ _1907,1909 , [O ii ] _λ _λ _3726,3729 , [Ne iii ] _λ _3869 , [O iii ] _λ _λ _4959,5007 , and H α , including a remarkable detection of the O iii Bowen fluorescence line at rest frame λ = 3133 Å. We analyze in this paper the joint NIRSpec + MIRI spectral data set. Combining six optical strong-line diagnostics (namely R2, R3, R23, O32, Ne3O2, and Ne3O2Hd), we find extreme-ionization conditions, with log _10 ([O III ] _λ _λ _4959,5007 /[O II ] _λ _λ _3726,3729 ) = 1.39 ± 0.19 and log _10 ([Ne III ] _λ _3869 /[O II ] _λ _λ _3726,3729 ) = 0.37 ± 0.18 in stark excess compared to typical values in the interstellar medium (ISM) at lower redshifts. These line properties are compatible either with an active galactic nucleus (AGN) or with a compact, very dense star-forming environment (Σ _SFR ≃ 10 ^2 –10 ^3 M _⊙ yr ^−1 kpc ^−2 and ${{\rm{\Sigma }}}_{{M}_{{* }}}$ ≃ 10 ^4 –10 ^5 M _⊙ pc ^−2 ), with a high ionization parameter (log _10 ( U ) =−1.75 ± 0.16), a high ionizing photon production efficiency $\mathrm{log}({\xi }_{\mathrm{ion}})={25.7}_{-0.1}^{+0.3}$ , and a low gas-phase metallicity (also confirmed by the direct, T _e method) ranging between 4% and 11% Z _⊙ , indicating a rapid chemical enrichment of the ISM in the past few megayears. These properties also suggest that a substantial amount of ionizing photons (∼10%) are leaking outside of GHZ2 and starting to reionize the surrounding intergalactic medium, possibly due to strong radiation-driven winds. The general lessons learned from GHZ2 are the following: (i) the UV-to-optical combined nebular indicators are broadly in agreement with UV-only or optical-only indicators; (ii) UV+optical diagnostics fail to discriminate between an AGN and star formation in a low-metallicity, high-density, and extreme-ionization environment; and (iii) comparing the nebular line ratios with local analogs may be approaching its limits at z ≳ 10, as this approach is potentially challenged by the unique conditions of star formation experienced by galaxies at these extreme redshifts.