In the opening post you wrote: "I'm editing 1080p video, x264 with crf=8 from a rawvideo camcorder stream at ultrafast preset, and using the default mpeg2 proxy preset for editing them."

187994 kb/s and 534776 kb/s, h.264 and 59.94fps are far away from the default mpg proxy settings. Or did I miss something?

He had asked for the ffprobe of the actual source clips I was working with, whcih is what I provided. Maybe I misunderstood what was being asked for?

Either way, whether I edit the source clip, or the low bitrate proxy, the same issue occurs. I am able to seek through the proxy much quicker, but the same crashing happens with proxy or with no proxy.

I'm compiling another nightly build to see what happens now.

Nightly builds won't help in this case. You should take the log/trace produced earlier and file a bug report at the MLT site.