Viewing Logs on Startup volume that freezes
I'm having the exact same problem with a client's 12,1 iMac.  I think it's hopeless for the iMac, but you might be able to put your MBP into relatively working condition if it has two graphics cards (you didn't mention what model you have).  If it does, then only one of them is busted (the AMD Radeon HD 6750M has come unsoldered), so you have to prevent the machine from trying to use it.  If you have an Intel HD Graphics 3000 card in there as well, you might be able to work off it.  Here's the GitHub guide:
https://gist.github.com/cdleon/d1eff7246a25193304284ecec40445b0
Here's the annoying (but insightful) YouTube video that led me to this discovery:
https://youtu.be/iLft4N54zvU
Check out both, give it a try, and please let me know how it goes.