I am now having the same problem
I don’t understand when you say “H.264 is not an FCP native sequence format”. In the past, I have been able to add a 720×1080 24p H.264 clip to a new sequence and have it play back in the sequence window with audio. Now all of the sudden, the video will playback, but the audio needs rendering. This happens even if I create a new sequence and match the settings to the clip.
I can not get anything to playback without rendering the audio anymore.