I've studied Vajrayana, so it may vary per school.

Nothing wrong with self-effort from what I've been taught. The idea with mindfulness if that if you are just aware of your thoughts, but don't do anything, it's not that helpful. It's like if someone is in a gutter and is mindful about that, they are still in a gutter. That can help them want to make a change, since that experience is probably not very pleasant, but the point is to make a change.

The waking up process can bring up stuff, for sure, but the idea is to bring a positive mind the whole time. This is how you go through the stuff and don't just get stuck in it, you go through it.

As far as I understand, past thoughts and actions build up karma which is basically the path folks will head down, since at that point it becomes automatic. That is the default state of being for those people. If they just go with it and don't try to change, they will do more of the same.

Not everyone wants to get to the advanced stages of meditation, which can be intense. Some basic meditation and mindfulness can be used to improve peoples' lives though. Even just basic mindfulness, like what is taught by Chade-Meng Tan at Google, can be helpful. It all depends on what the individual wants. I'm not familiar with the Diamond Approach, but like I mentioned in my first comment, I think whatever people find that brings positive results in their lives is probably a good thing.

It's posts like the article where I feel like certain spiritual teachers and schools are doing something wrong, if it's leading to someone having such a serious break down. That person might have had underlying issues, but as far as I can tell, meditation is pretty safe. Going farther on the spiritual path can be intense, and it's not for everyone, but what you learn in the basic practice helps you along the way so that you don't lose it when you get to the more advanced stages of meditation.

You also mentioned non-duality which is a more advanced subject, i.e. looking at things as neither positive or negative. The way I've learned basic meditation and mindfulness is to try to think less, slow down your thoughts, be more aware of what you're thinking and feeling, and replace negative thoughts with positive thoughts. This is pretty safe for beginners and the basics lay down the framework for more advanced meditation. I think it's good to hold off on the advanced subjects until people have a solid basic practice, but to each their own.

I have found that there is some use in the beginning in terms of looking at things as positive or negative, so people can clean up their lives and minds, which allows for deeper practice as they progress. The end goal is to overcome all karma and go beyond good and bad, but I don't think people can be expected to do that right away, it takes practice. If people can even just think a little more positive, be more aware of everything that runs through their head, and maybe be a little less reactive with others, their lives will be better and likely their relationships, as well. That's probably pretty safe for the general public to try to do, if they want to. The more hardcore spiritual stuff is not for everyone (most people don't actually want it, they just want to feel a bit better, which is great, too).