  [Abridged] The X-ray broadband spectra of the bare AGN Mrk 110, obtained by
simultaneous XMM-Newton and NuSTAR observations (Nov 2019 and April 2020), are
characterised by the presence of a prominent and absorption-free smooth soft
X-ray excess, moderately broad OVII and Fe Kalpha emission lines, and a lack of
a strong Compton hump. While relativistic reflection as the sole emission is
ruled out, a simplified combination of soft and hard Comptonisation from a warm
and a hot coronae, plus mild relativistic disc reflection reproduces the data
very well. We aim to confirm the physical origin of the soft X-ray excess of
Mrk 110 and to determine its disc-corona system properties from its energetics
using two new sophisticated models: reXcor and relagn, respectively. At both
epochs, the inferred high-values of the warm-corona heating from the X-ray
broadband spectral analysis using reXcor confirm that the soft X-ray excess
originates mainly from a warm corona rather than relativistic reflection. The
intrinsic best-fit SED determined at both epochs using relagn show a high X-ray
contribution relative to the UV and are very well reproduced by a warm and hot
coronae plus mild relativistic reflection. The outer radii of the hot and warm
coronae are located at a few 10s and ~100 Rg, respectively. Moreover, combining
the inferred low Eddington ratio (~ a few %) from this work, and previous
multi-wavelength spectral and timing studies suggests that Mrk 110 could be
classified as a moderate changing-state AGN. Our analysis confirms the
existence of a warm corona as a significant contribution to the soft X-ray
excess and UV emission in Mrk 110, adding to growing evidence that AGN
accretion deviates from standard disc theory. This strengthens the importance
of long-term multi-wavelength monitoring on both single targets and large AGN
surveys to reveal the real nature of disc-corona system in AGN.
