  The Inverse Seesaw mechanism is dynamically realised within the Minimal
Lepton Flavour Violation context. Lepton number, whose breaking is
spontaneously realised, is generalised to a global Abelian factor of the whole
flavour symmetry, that also plays the role of the Peccei-Quinn symmetry. The
associated Goldstone boson is a Majoraxion that solves the Strong CP problem
and represents a Dark Matter candidate.
  Three distinct scenarios are identified in terms of flavour symmetry and
transformation properties of the exotic neutral leptons that enrich the
Standard Model spectrum. The associated phenomenology is studied, focusing on
the deviations from unitarity of the PMNS mixing matrix. The strongest
constraints arise from the determination of the number of active neutrinos
through the invisible width of the $Z$, the comparison of the measured $W$
boson mass with its prediction in terms of the Fermi constant from muon decay,
and the null searches for the radiative rare muon decay and $\mu\to e$
conversion in nuclei. The heavy neutral leptons may have masses of a few TeV,
leaving open the possibility for a direct detection at future colliders.
  The impact of the recent measurement of the $W$ mass at the CDF II detector
has also been considered, which, in one of the scenarios, points to a sharp
prediction for the masses of the heavy neutral leptons at about $2-3$ TeV.
