not sure if it is worth it to buy/replace just the mobo when a working unit with damaged lcd screen is on sale for less.  LCD passing test has no bearing on pc ability to pass POST.  looks like you have done all you can. 
Got this Laptop off of a friend who said I can have it if I fix it. She said one day it started doing that and hasn't worked since. The fans turn on, and theres nothing, but if I do the LCD BIST it starts and runs normally, no problems. If I connect it to an external display theres no output either. I've reset the cmos, swapped out RAM, almost everything short of replacing the motherboard. Just want to see if there's anything else I can do before replacing the mobo.
Latitude E7250 turns on and passes LCD BIST but wont go passed that