I’ve spent the last 2 days troubleshooting my neighbor’s MBP late 2011 with the same problem after they had it in the repair shop 3 times for $500. I got it working enough to get everything off of it by using the fsck and after “volume appears to be ok”, typing “reboot” (this was the only way to get it to boot up normally as none of the other key prompts would work). I then backed it up to an external hard drive, wiped the current hard drive using target disk mode with another MBP also running High Sierra (I had to install the update to that computer first as disk utility had a glitch before the recent update), and then made a usb boot disk of High Sierra (all very time consuming). After the reinstall, which took forever and stopped several times, and then doing the same recent update which was not on the usb install, it worked great— until I turned it off and closed it up and my neighbor took it home. When they opened it up, same white screen! They’ll bring it back today and I’m planning on opening it up to see if possibly one or more cables is pinched and causing some of the troubles. I’m pretty sure the clean install helped as it restarted 20 times easily after I finished. The search on 2011’s and SSD drives leads me to believe that cable may be the problem which makes a lot of sense.