Aha! I have figured out why Paul's suggestion did work. Converting to Triax to fix the morphs, and use these to replace the existing ones won't work because legacy and Triax morphs use different file formats. It was misleading at first because the file extension (DSF) is still the same, but one is binary (proprietary) and the other is plain text (JSON standard). One cannot be used to replace the other. This is rather surprising, because when we save the figure as DUF, we would expect the morphs to follow the new standards, but instead they are saved in the old format. This mixture of new and old file formats is what makes it impossible to save morph changes, because a "Morph Asset" in DS4 can only be in JSON format. This is messed up.
I have also tried exporting a fixed CR2 from DS, but it gives me errors, claiming it doesn't find the OBZ file, while I didn't save it as compressed, so there is no way to make it find it. When I load the figure back, it doesn't show on scene.
On the bright side, I've managed to keep trying to create the negative range ERC and at some point it didn't crash DS. I didn't do anything different, it just didn't crash DS that time - but then there was no way to save the changes. I have searched online, and found other people who had a similar situation with non-TriAx content where they couldn't save any morph changes - so this isn't new.
The bottom line is that saving the head in the new DUF format won't save the morphs in the new JSON format. That's why changes cannot be saved.