Among the nearest most metal-poor starburst-dwarf galaxies known, SBS 0335-052E is the most luminous in integrated nebular He II.4686 emission. This makes it a unique target to test spectral synthesis models and spectral interpretation tools of the kind that will be used to interpret future rest-frame UV observations of primeval galaxies. Previous attempts to reproduce its He II.4686 luminosity found that X-ray sources, shocks, and single Wolf-Rayet stars are not main contributors to the He IIionizing budget; and that only metal-free single rotating stars or binary stars with a top-heavy IMF and an unphysically low metallicity can reproduce it. We present new UV (COS) and optical (MUSE) spectra that integrate the light of four super star clusters in SBS 0335-052E. Nebular He II, [CIII], CIII], C IV, and OIII] UV emission lines with equivalent widths between 1.7 and 5 A and a CIV lambda lambda 1548, 1551 P-Cygni like profile are detected. Recent extremely metal-poor shock + precursor models and binary models fail to reproduce the observed optical emission-line ratios. We use different sets of UV and optical observables to test models of constant star formation with single non-rotating stars that account for very massive stars as blueshifted OV lambda 1371 absorption is present. Simultaneously fitting the fluxes of all high-ionization UV lines requires an unphysically low metallicity. Fitting the P-Cygni like + nebular components of CIV lambda lambda 1548, 1551 does not constrain the stellar metallicity and time since the beginning of star formation. We obtain 12+log(O/H)= 7.45 +/- 0.04 and log(C/O)= -0.45(-0.04)(+0.03) for the galaxy. Model testing would benefit from higher spatial resolution UV and optical spectroscopy of the galaxy.