I tried to do this, install hashcode's CM 11.0, and
set up dual-booting with a modded stock ROM. I did finally get it working, just from the info here
, and wiping everything a few times.
I installed TWRP 18.104.22.168 through fastboot just fine.
I did run into the problem with mounting /data, but not until after I had CM 11.0 installed and running. I had to go through the dual-boot set up in KFU a couple of times before I could get the alternate ROM to boot properly. I don't quite remember what steps I actually took that finally got everything working correctly.
( I tried to reply to hachcode's thread, but I realize I haven't made enough posts to be able to post in the dev forum. I've just been lurking here for a year or so).