  In addition to their jet-like dynamic behaviour, spicules usually exhibit
strong transverse speeds, multi-stranded structure and heating from
chromospheric to transition region temperatures. In this work we first analyse
\textit{Hinode} \& \textit{IRIS} observations of spicules and find different
behaviours in terms of their Doppler velocity evolution and collective motion
of their sub-structure. Some have a Doppler shift sign change that is rather
fixed along the spicule axis, and lack coherence in the oscillatory motion of
strand-like structure, matching rotation models or long wavelength torsional
Alfv\'en waves. Others exhibit a Doppler shift sign change at maximum
displacement and coherent motion of their strands, suggesting a collective MHD
wave. By comparing with an idealised 3-D MHD simulation combined with radiative
transfer modelling, we analyse the role of transverse MHD waves and associated
instabilities in spicule-like features. We find that Transverse Wave Induced
Kelvin-Helmholtz (TWIKH) rolls lead to coherence of strand-like structure in
imaging and spectral maps, as seen in some observations. The rapid transverse
dynamics and the density and temperature gradients at the spicule boundary lead
to ring-shaped \ion{Mg}{2} k and \ion{Ca}{2} H source functions in the
transverse cross-section, potentially allowing IRIS to capture the KHI
dynamics. Twists and currents propagate along the spicule at Alfv\'enic speeds,
and the temperature variations within TWIKH rolls produce sudden appearance /
disappearance of strands seen in Doppler velocity and in \ion{Ca}{2} H
intensity. However, only a mild intensity increase in higher temperature lines
is obtained, suggesting there is an additional heating mechanism at work in
spicules.
