The latest super experimental build for 6d is in the post above your post.
This build also has SD-UHS hack module in it.
I believe this build has the SD-UHS module, when activated in module tab, it automatically loads at each camera startup, so you won't even notice it's there. (You can check in ML menu on the debug tab there is a menu item in there that shows overriden memory or something, if you look there you will also see override stuff from SD-UHS hack)
SD-UHS doesn't work with all memory cards, it sets the SD card writer in camera to a specific high write speeds SD UHS I specification setting.
It does work perfect with Sandisk extreme pro UHS-I U3 cards, the types which mention 95MB/s or 170MB/s on the card.
If your card doesn't support the settings used in SD_UHS hack, the camera writes as the same speed as before, or sometimes even much slower.
Trying to answer some of your questions:
1. What is the aim of the crop_rec mode? I'm assuming this uses a smaller region of the sensor? Was it created to help avoid aliasing? Just trying to understand what the use of this mode is compared to the normal mode.I don't think there was a goal or aim from the beginning, crop mode just happens to be a mode already available, and people discovered cool usages for it.
Crop mode is something that is standard available in Canon cameras, Canon uses it for 5x zoom live view.
Most Canon camera's can't record video while in 5x zoom/crop mode, the 600d and 60d however had some sort of option to record video while in zoom mode.
So somebody from the ML developers soon found out you can also record video in this mode on all other camera's ( Where canon didn't give you that option standard)
This option actually records at 1:1 pixel readout, so it actually gives you a 3x zoom (because it only uses 1/3 portion of your sensor, instead of the whole width)
Later on, ML developers found out that you could increase resolution while in this crop mode.
This gave birth to the high resolution crop modes Alex introduced on April 1 few years ago.
Because crop mode is in 1:1 pixel readout, it has almost no aliasing compared to normal full sensor view mode.
2. Is it possible to get decent slow motion with the 6D (without too much aliasing)?, if so which mode/resolution should I be using? The build in the post above has a 50fps crop mode, which has the same amount of aliasing as normal non slowmotion video on 6d.
The 50 fps mode has to be used with the camera in 720p50 mode, in normal 1080 mode the camera freezes in 50fps mode and you need to pull out the battery to fix it again
Also vertical resolution is limited to about 800 pixels.
3. How do I get the SD UHS hack on the 6D? Download the ML build from the post above
4. Which is the latest and greatest crop_rec module for the 6D? Currently running the latest 6D crop-rec build from the experimental builds page. The one in the build in the post above.
P.S.
The build 2 posts above is a merge with Danne's build, mostly aimed at eos-m.
There is a submenu in crop_rec (press Q button while on crop menu).
There are many options there, which aren't integrated with 6d, so most options do nothing, the most useful option does work, that is lower bitrate option, so you can use 12-bit or 10-bit option in crop_rec submenu.
