Looks like the mp4 import support is highly flaky. Known good files crash it easily. I’ve down-graded my Quicktime version to 7.1.6 (which they suggest as the minimum, which is probably what the devs had at the time), and converted my .mp4 to .mov (without re-encoding via ffmpeg -i -vcodec copy -acodec copy ). Then tried to import that .mov file and bingo! It works. However it’s slow as hell. Seeking around the timeline is way slower than having say, PAL DV footage in the timeline.
But slow is better than immediate crash, so I ran with it. Next time I’m going to keep the original DV footage around longer *smacks forehead*.