This is what I get with -current /bin/sh and your patch:
...
[ 10.2521573] init path (default /sbin/init):
[ 10.2827453] init: copying out flags `-s' 3
[ 10.2827453] init: copying out path `/sbin/init' 11
[ 10.2827453] entropy: best effort
Enter pathname of shell or RETURN for /bin/sh:
[ 50.1619768] Alignment error: pid=31.31 comm=sh pc=4099a12c
[ 50.1619768] dfa = 00000000e0002000
[ 50.1619768] dfc = 0000000000000000
[ 50.1619768] pid 31 (sh), uid 0: exited on signal 10 (core not
dumped, err = 30)
It doesn't get as far as a single user mode. I think the patch made
things a bit worse, since the kernel now hangs and doesn't even drop
into debugger.
If the issue is confined to the kernel then the older /bin/sh
binary should not make any difference as it still complies with V9 ABI
and should be backward compatible with NetBSD system libraries.
It's a bit easier to debug in single user mode.