Stellar population synthesis models are at the heart of essentially all inference about galaxies. As observations have steadily pushed the redshift limit deep into the reionization era and mass limits at intermediate redshift down into the dwarf galaxy regime, these models have begun to show signs of strain. Nearby star-forming dwarf galaxies represent one of our only avenues towards testing and refining these models at sub-SMC metallicities (<20% Zsun). I will present results from our HST/COS and MMT campaign targeting both high-ionization nebular and stellar wind lines in highly star-forming dwarf galaxies, probing the ionizing spectra and outer atmospheres of very metal-poor massive stars. Nebular He II emission in these spectra reveal a striking disagreement with model predictions, and I will present new work exploring the possibility that this emission is a signature of high-mass X-ray binaries or peculiar products of binary evolution. Finally, I will preview new HST programs which will soon provide the first clear views of stellar winds below 10% solar metallicity and lay the groundwork for empirical calibration of stellar models in this regime.