Did you do 'install' or 'upgrade'?
Doing an overlay 'install' (ie without any mkfs) might work better!

>  * I have non-standard partitioning on my laptop.  I didn't want to recreate
>    it, so I went with "use existing" (I think that this was during an
>    install; during a merge of /etc after an initial install, I botched
>    something and decided just to re-install and restore /etc from backup).

The way /etc is handled is broken!

>    The sysinst tool apparently did NOT mount /dev/wd0a, and instead
>    quickly filled up the /mnt *directory* (on ramdisk?), producing a
>    stream of errors.

mmm which version of sysinst were you running?
/dev/wd0a should (now) have been mounted of /targetroot
Anything that assumes /mnt needs fixing.

>    I killed the process, cleaned up the the files, restarted the
>    install, but used ^Z to suspend, and manually mounted, just before
>    sysinst began to unpack.  Everything seemed okay, then.
> 
>    There wast at least one other sysinst oddity, though.  I asked
>    it *not* to install the etc.tgz.  It still moved my old /etc
>    to /etc.old.

That has to be the old sysinst.
The one in 'current' only saves /etc if etc.tgz is requested.

>    (Actually, I do not now remember if it was "update", or
>     "install", or just "unpack sets" that I selected when the /etc dir
>     was incorrectly relocated.)

Only 'update' tries to save /etc


-- 
David Laight: david@l8s.co.uk