Can I boot from a usb using a secondary /external display?

Hmm, tricky one.

The specifics of this vary laptop by laptop, so I can't be sure, but I can give you some general things to try.

First up is 'just try it' - connect an external display and try to boot, see if anything pops up. Least effort involved, but also least likely to work. Still, it would be silly to try a bunch of difficult invasive stuff only to find out you could have done it easy.

Next up would be to see if the laptop will run and try to boot with the lid closed: some models will recognize the combination of 1: laptop closed and 2: external display connected and conclude you want to use the laptop with a docking station like setup, and will work. Make sure to also connect an external keyboard and mouse.

No joy there?

After that things get invasive, ie. we need to do some hardware work. You could, at this point, opt to replace the screen. Alternative is to unplug the screen and then try to boot with an external monitor connected, hoping the BIOS understands the integrated screen is not connected and the external is, and is well-programmed enough to decide the external is your primary.

That's all I've got.