external monitor only, stuck in safe mode

MBP running Mojave 10.14.6. The built-in display is broken but I've got it connected to a Dell LCD over HDMI which mostly works great.

My problem is I installed the latest round of security patches for 10.14.6 and something went wrong during the update. Rebooting brings me to an error screen:

"MacOS could not be installed on this computer.

The installer resources were not found."

and then a RESTART button at the bottom of the window. Pretty sure I've seen this sort of behavior before, and the internet confirms that clicking restart should allow the installer to sort itself out and boot normally.

The problem is, because of the broken laptop display, I can only see the error screen with the clamshell closed, and I can't click the RESTART button with the clamshell closed. I have a USB keyboard but no USB mouse (I assume that would fix the problem but it might be a day or two before I can track down a USB mouse). Opening the clamshell forces the error message back to the broken display and the external monitor is only a grey background; I can move the arrow around with the trackpad but can't interact with the (hidden) error dialogue.

I found I can hold shift when booting and can get into safe mode just fine. Booting with Command-R lands me in Recovery mode but it's the same situation where I can only see with the window while the clamshell is closed, and it is not responsive to anything on the external keyboard. I tried holding Option when booting, but am unable to see the boot volume menu even with the clamshell closed.

Sooo, my question is: how do I fix this from Safe Mode? I tried running First Aid from Disk Utility which didn't have any effect. Is there a way I can redownload/rerun the patch from safe mode, or some other way to untangle this?