  The Jiangmen Underground Neutrino Observatory (JUNO) project aims at probing,
at the same time, the two main frequencies of three-flavor neutrino
oscillations, as well as their interference related to the mass ordering
(normal or inverted), at a distance of ~53 km from two powerful reactor
complexes in China, at Yangjiang and Taishan. In the latter complex, the
unoscillated spectrum from one reactor core is planned to be closely monitored
by the Taishan Antineutrino Observatory (TAO), expected to have better
resolution (x 1/2) and higher statistics (x 30) than JUNO. In the context of
neutrino energy spectra endowed with fine-structure features from summation
calculations, we analyze in detail the effects of energy resolution and nucleon
recoil on observable event spectra. We show that a model spectrum in TAO can be
mapped into a corresponding spectrum in JUNO through appropriate convolutions.
The mapping is exact in the hypothetical case without oscillations, and holds
to a very good accuracy in the real case with oscillations. We then analyze the
sensitivity to mass ordering of JUNO (and its precision oscillometry
capabilities) assuming a single reference spectrum, as well as bundles of
variant spectra, as obtained by changing nuclear input uncertainties in
summation calculations from a publicly available toolkit. We show through a
chi-squared analysis that variant spectra induce little reduction of the
sensitivity in JUNO, especially when TAO constraints are included. Subtle
aspects of the statistical analysis of variant spectra are also discussed.
