ino64: desastrous update - recommendations not working!!!

On almost every CURRENT that has been updated according to UPDATING
entry 2017-05-23 regarding ino64, the recommended update process ends
up in a desaster or, if the old environemnt/kernel is intact, itr
doesn't work.

Procedure:

make -jX buildworld buildkernel [successful]
make installkernel [successful]
reboot
Booting single user mode as recommended withnthe newly installed kernel
BUMMER!
When it comes to the point to type in the full path of /bin/sh, /bin/sh
immediately fails with SIGNAL 12

In this case, I can boot without problems the old kernel and the system
works again.

But, depending on the entry revision from which I started the 22nd, or
23rd of May ino64-deal, there is a more harsh failure!

According to the above recommendation of updating, BUMMER! doesn't
occur at that point and the shell /bin/sh starts as expected.
Performing 

mergemaster -Fp

also performs well without any questions or installations so far,
but then

make installworld

BUMMER! again and this time with fatal consequences! The installation
fails in libexec/rtld-elf or something like that in the
source/object tree after copying libexec/ld-elf.so.1. I
see /libexec/ld-elf.so.1 successfully copied with the security copy
marked with appendix .old being of a conclusive date and time.
The installworld bails out, leaves the tree in a mixture of old and new
binaries and now, thanks, the whole system ist wrecked.
When trying to reboot such a half-ready installation in single user
mode, I can't even get an shell enymore.

How can I fix this emergency case with the tools aboard?

Since there is no compiler or build infrastructure any more on the USB
bootimage, I can not simply installworld and installkernel - the boot
image is useless - on this list I had such a discussion in March. For
short: I have the intact and complete /usr/obj tree and I think it
would be a great deal to be able to simply boot via USB memstick and
perform installworld with propper settings of DESTDIR= and sibblings.

Yes, now what is to do ... :-(

Help appreciated and thanks in advance for those reading so far.

O