This is one of the issues you face with a headless MacBook system. As you can't access the settings from the internal display as the system still thinks it's there!
Depending on your external display you might be able to lower its resolution setting so you can get past the startup banner which is still on the screen as the system is trying to jump to the higher refresh rate. Here's a deeper breakout from EveryMac listing of your system:
13.3" color widescreen TFT LED-backlit active-matrix "glossy" display with a 1440 by 900 native resolution. Apple reports that it also supports "1280 by 800, 1152 by 720, and 1024 by 640 pixels at 16:10 aspect ratio and 1024 by 768 and 800 by 600 pixels at 4:3 aspect ratio".
The other option is if you have access to a second system which you can remotely administer the systems settings over a network connection.
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
0