How can the TrackPoint driver be changed from default libinput to a newer libinput or back to Synaptics?

For HWE installations the command would be

sudo apt install xserver-xorg-input-synaptics-hwe-18.04

You can switch to synaptics driver by removing /usr/share/X11/xorg.conf.d/XX-libinput.conf file.

So following from the suggestions of @Hi-Angel I have upgraded libinput from the default and this appears to have improved things a great deal. The procedure was as follows:

sudo apt install check doxygen libev-dev python3-sphinx
sudo pip3 install recommonmark sphinx_rtd_theme    
git clone https://gitlab.freedesktop.org/libinput/libinput
cd libinput
meson --prefix=/usr builddir/
ninja -C builddir/
sudo ninja -C builddir/ install
sudo systemd-hwdb update

This is followed by a reboot.