It sounds like you've covered most of the basics, but you may want to check the GPU and display data lines for issues, as well as the TCON board if your model has one. Sometimes, cold solder joints or GPU issues can cause display problems, even when external displays work. Also, resetting the SMC and PRAM might help. For voltage checks, typical values for the MBP A1278 display include VCC ~12V, VLED 12V, VGL ~-5V, and VGH ~15V. If everything is correct, it could be a logic board issue.