thanks Doktor5000, I will see if I can get it to start by extrapolating from your earlier suggestions.
mailing list is always available too.
ITA84: I have over the last few months been creating cauldron systems.
first were unsuccessful upgrades of Mga6 in VM.
when they became regularly successful, I "upgraded" my tests to bare metal upgrades.
When they became successful regularly, I have moved on to net-installs from scratch on bare metal.
earlier trials would only intermittently identify keyboard or mouse, so would halt at language selection page.
later ones would have 'script errors' or file conflicts, and so would not complete installation.
these latest ones are the ones I am having problems with, so things are progressing quite well, as the installation at least completes.
they do not have a prefdm file at all, (and quite some others when compared with a fresh M6 install) but can be started with "startx start_desktop_choice_of_installed_desktops", so are at least partly usable.
If I copy the prefdm (and some others) from a working "upgrade" cauldron, still no dm greeter, but an improvement at least (a moving mouse "block") where one would expect the dm greeter.
more trials to continue