There is no headphone jack on the 6D2. That cable requires Magic Latern FW and is intended for the T2i and 60D. ML firmware is not (fully) supported on the 6D2. Can't say if it would work since the ML website states porting only. If you brick it, it will void your warranty. My advice is don't do it, (don't even try) but thats up to you.
Sorry I don't have something better to offer. The 6D2 does decent 2K video, and can be further enhanced using a Rode or wireless equivlemnt lappel mic, but monitoring is not an option.
You are looking at another body for this feature. I'd wait a bit as things are going to shift in the near future. R5 release, R6 announcement. Memorial Day is coming too, and that means big sales.