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Once you get "cannot assemble drivers for root", it's over.
And I just failed the reading comprehension test. Sorry, that was not the case you were asking about:
After creating/formatting a disk in single-user mode, I reboot and start the install. It copies the installer to /dev/c0t0d0s1 and reboots. On startup, it gets to "Running in command line mode" then seems to endlessly do nothing.
Is how long should I wait before I can safely assume it isn't going to continue/succeed?
I can't directly answer the "how long" question, but the above seems to indicate that you might have your console directed to somewhere unexpected. Are you expecting the output to come to the video console or the serial port? Be aware that Solaris doesn't always make the same decision OBP does on this.