Help! Howto installworld crashed system with USB image?

Within the past several hours, FreeBSD crashed due to serious bugs and
some boxes of ours hang with the uncomplete workaround with
EARLY_AP_STARTUP. During a recompilation and
installworld/installkernel, one of my workstations suddenly crashed and
spontaneously rebooted. After that, the loader complained about "not
kernel" and left me alone at the "OK " prompt of the bootloader.

After some fast investigations I realized, that except /bin/sh all
files on the SSD (Samsung 850 PRO, crashed kernel had NANDFS option
enabled as well as device nandfs, if this is of interest, but I doubt
it). Since the whole SSD is so far intact including the /usr/src
and /usr/obj and with only the binary and ölibraries (probably, not
confirmed) corrupt, I tried to rescue via using the most recent
12-CURRENT ISO FreeBSD USB image

FreeBSD-12.0-CURRENT-amd64-20170105-r311461-memstick.img

But I'm lost here! I mounted for convenience usr/obj and usr/src
onto /usr/obj and /usr/src respectively onto the USB mounted filesystem.

Everything else of the SSD is mounted onto /mnt. 

I thought I could simply "bootstrap" an installworld with the toolchain
resident on /usr/obj, but I fail in a painful way.

cd /usr/src,
make DESTDIR/mnt installworld installkernel

bugs out with some mysterious error telling me to set COMPILER_TYPE=,
so I did set this variable to cc.

The result: I figured out that the USB image is one of the useless
minimalistic ones with no compiler aboard. Fine. No rescue, no cc, no
nothing.

I desperately need some advice in how I can perform installworld and
installkernel.

I have a customized /etc/src.conf and /etc/make.conf, so I guess I have
to set

ETCDIR=/mnt/etc

also. Since I use a different name of my kernel (not GENERIC), I also
need to set KERNCONF and KERNEL, so I guess, with KERNCONF in question,
since I have already a kernel ready to install.

But how can I delegate the installation procedure to use anything
from /usr/obj including the compiler?

Something has changed to the worse in FreeBSD! I remember that I had a
similar situation a while ago last year on 10 or 11-CURRENT, where a
crash destroyed libraries and I was capable of rescueing the system via
the USB image and installworld. 

Either some great mind erased the necessary compiler from the (too)
minimalistic image, or something new has been introduced to perform a
rescue/standalone-bootstrap installation. 

Either way, I would be really happy if someone could give me a hint how
to rescue the broken system.

Thanks in advance,

Oliver


p.s. I've already written another mail to the list with a more unclear
subject, I hope this subject makes it more clear and after the anger
has gone away, I think I can express the situation more clearly.