  Time variability of astronomical sources provides crude information on their
typical size and on the implied physical mechanisms. PKS1830-211 is a
remarkable radio-bright lensed quasar with a foreground molecular absorber at
z=0.89. Small-scale morphological changes in the core-jet structure of the
quasar -- which is magnified by the lensing -- result in a varying illumination
of the absorber screen, which in turn causes variations in the absorption
profile. We aim to study the time variations of the system [...] in order to
obtain constraints on both the quasar activity and small-scale structures in
the ISM of the absorber. We used ALMA to monitor the submm continuum emission,
together with the absorption spectra of the H2O and CH molecules, with 17
visits spread over six months in 2016. [...] From the continuum data, we
followed the evolution of the flux density, flux-density ratio, spectral index,
and differential polarization between the two lensed images of the quasar; all
quantities show significant variations related to the intrinsic activity of the
quasar. We propose a simple parametric model of a core plus a ballistic plasmon
to account for the continuum evolution, from which we constrain a time delay of
25+/-3~days between lensed images. The spectral lines reveal significant
variations in the foreground absorption. A PCA highlights apparent wavy time
variations, possibly linked to the helical jet precession period of the quasar.
From the deep averaged spectra towards the SW image, we detect the absorption
of 13CH and estimate an abundance ratio of 12CH/13CH~150. We also measure the
oxygen isotopic ratios, 16O/18O=65.3+/-0.7 and 18O/17O=11.5+/-0.5. Finally, we
find a remarkable continuous shallow trough in the water absorption spanning a
velocity interval of nearly 500 km/s. This broad absorption could be the
signature of an extra-planar molecular component. [Abridged]

Astrophysics of Galaxies