From what I can find upgrading a laptop screen isnt really a "thing", they are designed with the VGA/HDMI outputs for people wanting better screens etc. Each display panel (laptop screen,desktop monitor, phone screen, whatever) has its own controller chip and default configuration for the initial behaviour, but not exactly a standard across all manufacturers. eg a recent arduino project I put together had an lcd sreen and displaying text it did so in japanese until I researched the panel, found the command needed to get it to use the english character set, even then i had to do display more special characters which it stored in a different character set. So it could be the laptop hardware is talking the wrong language to the panel, nothing to do with bios or windows. For a panel intended for a monitor it makes sense the common resolutions share the same command (more mine i had to convert a binary chart to hex values, not as simple as just telling it "english" or in your case a resolution number). If the laptop was in a range that came with different panels then the control might allow you to switch panels successfully. In your case the lappy may predate UXGA so maybe not so lucky.
Really dumb question: why not get a serial to usb converter (not adapter cable). It emulates the port for those programs that talk directly via serial. I had to use one for some telescope gear as my laptop with serial died and the only ones I have left are too ancient to run the software.
Can't recall the brand off hand but works a treat.
Disclaimer: dont read further unless you are prepared to buy a replacement lappy if it goes wrong.
I havent tried this and if it blanks your screen in an unusable resolution I take no responsibility. But
hacking the registry might do the trick.
I'd do this with the external monitor connected and make sure you can switch to it and back easily at any time. if you lock the laptop into a resolution it doesnt like your only hope is to use the external display to help you edit your way back out again. Take a backup/export the registry key(s) you change so you can double click the *.reg file and put things back. Possibly via safe mode.