Personally, I always do this (the most straightforward & path of least resistance), no, its not AAF (PITA in general, IMO):
Both Final Cut and Resolve can export a ‘multichannel’ video file. If the audio tracks in FCPX or Resolve are set up with dedicated lanes, then exactly these lanes will export as multichannel audio within the video file. In Resolve there is more control (eg, 24 bit PCM settings etc) but FCPX works fine as well - just a bit more hidden as is usual for MacOS (!). That requires picking though the ‘share’ menu (as Apple FCPX calls ‘export’). On the last page of that process, there is a pull down menu for multichannel audio.
In Nuendo then, is very straightforward: Create a project, then import the video. This will then extract all of the audio tracks and put them on individual lanes. Mix. Later, bounce what you need (eg: mastered stereo, mutlichannel stems, or even bounce bask to video, albeit with only one bounce format for now).
Hope that helps. May not exactly fit the workflow depending on what your client wants back. In my case, at this stage am usually looking for a mastered stereo audio track (-16 LUFS for music vid, maybe a different spec for your client), and then just drop that back into the FCPX or Resolve project for final video render.