The ultrafaint dwarf galaxy ReticulumII was enriched by a single rare and prolific r-process event. The r-process content of ReticulumII thus provides a unique opportunity to study metal mixing in a relic first galaxy. Using multi-object high-resolution spectroscopy with VLT/GIRAFFE and Magellan/M2FS, we identify 32 clear spectroscopic member stars and measure abundances of Mg, Ca, Fe, and Ba where possible. We find 72_-12_^+10^% of the stars are r-process-enhanced, with a mean <Ba/H>=-1.68{+/-}0.07 and unresolved intrinsic dispersion {sigma}[Ba/H]<0.20. The homogeneous r-process abundances imply that RetII's metals are well mixed by the time the r-enhanced stars form, which simulations have shown requires at least 100Myr of metal mixing in between bursts of star formation to homogenize. This is the first direct evidence of bursty star formation in an ultrafaint dwarf galaxy. The homogeneous dilution prefers a prompt and high-yield r-process site, such as collapsar disk winds or prompt neutron star mergers. We also find evidence from [Ba/H] and [Mg/Ca] that the r-enhanced stars in RetII formed in the absence of substantial pristine gas accretion, perhaps indicating that ~70% of RetII stars formed after reionization.