Answer to "Boots in safe mode but hangs at blue screen otherwise"It's the graphics chip (GPU) it goes bad and you can only boot in safe mode, there are some tricks you can try on YouTube, also it's not a hard fix for any computer repair shop, buy a new nividia gpu for your MacBook Pro model on eBay and have a shop do it for you, I'm guessing under 100 to fix.
Answer to "Only wants to boot in safe mode"Graphics card is going bad, once it goes bad it will only boot in safe mode. There is some tricks on you tube like heating up the graphics chip (gpu) with a heat gun or blow dryer. I got a computer with the same issue.
Answer to "Unable to install OS X or boot OX (rMBP A1398)"If it boots in safe mode reinstall the OS and try a different version of OS X, like if your on Mavericks try Yosemite instead. If it still does t boot replace the hard drive. And if it still doesn't boot with a new hd I got good news and bad news, the good news is you probably need a new cable that goes from the hd the mother board and the bad news is it could also mean your mother board is done.