I would consider anything you do with the pre-release profiles and pre-release DNG Converter to be experimental. The newest XRCCPP is older (2010) than the newest DNG Profile Editor (2012) and they do create slightly different looking profiles.
It does bother me that the XRCCPP software doesn't recognize the D750 DNGs made with anything other than the oldest compatibility option, though. I say this because I've never had a problem with the newer compatibility options so I'm not sure what is going on, there. I'd really appreciate getting a color-checker NEF from you, via Dropbox or some other way. If I can confirm this is an issue, then Adobe needs to know about it before they release the final versions of ACR 8.7 and DNGC 8.7 and I'll add my voice to yours in reporting it. Adobe would also want to see a sample Color Checker file that exhibits the problem. It's possible things work fine for them and not for you and there is some difference in the file they need to understand.
Once the final release versions of the DNGC and ACR are released, if you still have an issue, then contact X-Rite, like you have once, already, because it won't be a beta conversion, anymore.
If I was doing it, I'd use the newest DNGPE 1.0.4 and DNGs made with the newest compatibility option (7.1) and if that didn't work then I'd ask a question about it. Since you haven't tried anything besides the very oldest DNGPE beta the issues you're having may have been addressed in the Sept 2012 version. I realize you want to use X-Rite software like you've done in the past, and that's fine, I'm talking about now, during the RC period, before the final release.
I will be in the same situation once Adobe supports my new 7D Mark II camera, as I'll be wanting to create profiles for it, but right now not even the public release candidates at Adobe Labs support the camera as it was only available starting Oct 30, so I have to wait. It would be disappointing if there is something wrong with the ACR 8.7 and DNGC 8.7 conversions that make them incompatible with the DNGPE and XRCCPP software.