It should not be possible for one person to make a mistake of this magnitude. In case of a disconnected/off main safety system, the train should be limited on a hardware level to not be able to go over 40km/h.

If I read the description correctly, those hardware level systems were manually overridden, which should only be done in extremis. My guess is that this is going to turn into a normalization of deviance case, since such a cavalier attitude toward safety was unlikely to be one person's first time doing something so dangerous. There was no dilemma here and no time pressure.

Your suggestion about limiting speed after a shut down is a good one. On top of that, the rail operators should have spotted a trend of repeated safety system overrides and investigated why that was happening.

Exactly this was my first thought! I always thought highspeed trains operate under a model of clear sectors where one train is just allowed to enter when it was cleared by the previous train. From my layman point of view, those incidents point to a fundamental design flaw.