  Among the nearest most metal-poor starburst-dwarf galaxies known, SBS
0335-052E is the most luminous in integrated nebular He II {\lambda}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 {\lambda}4686 emission luminosity found that X-ray sources, shocks,
and single Wolf-Rayet stars are not main contributors to the He II-ionizing
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 which integrate the light of four super
star clusters in SBS 0335-052E. Nebular He II, [C III], C III], C IV, and O
III] UV emission lines with equivalent widths between 1.7 and 5 {\AA}, and a C
IV {\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 which account for very massive stars, as blueshifted
O V {\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 C IV {\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\pm0.04 and log(C/O)=-0.45(+0.03)(-0.04)
for the galaxy. Model-testing would benefit from higher spatial resolution UV
and optical spectroscopy of the galaxy.
