I have tried several solutions for retargeting the CMU bvh and mixamo animations to several different skeleton types, including the diffeomorphic route. I found the only solution that really works for me is, oddly, the rokoko blender plugin retargeter.
Now a bit of warning first, unless you modify the rokoko plug-in code, it requires you to create an account and login once, meaning you have to put blender online for a second. no big deal, and as far as i know, it only makes you login online the one time for counting users purposes. i have been asked by rokoko not to share the code to bypass login anymore, so, i won't. again, no big deal. it seems to be a counter process and not a telemetry process. But, and this is the kicker, you need to use the 1.1.0 version and not update. if you use the 1.1.1 version, or allow 1.1.0 to update, the root bone with stay in place while the rest of the armature go where it is supposed to. this results in some rather disturbing looking actions
(i have filed a bug report on their github about this)
here is a link to the rokoko blender plugin
https://github.com/Rokoko/rokoko-studio-live-blender/releasesAlso, once you get everything working, i recommend exporting a bone list so you shave time off of future processes.
Exporting a bone list is different than the 'save' route available in the plugin. if you use the save route, and have a mistake in the bone retargeting layouts, it will reproduce everytime you try to use it. which can be fixed by deleting a file it creates in the plugins directory of blender. so, until you are sure you have it perfect, export instead of save the bone list. (you should be able to see what i mean when you go to use the plugin)