Thanks for your questions. Here are some of my thoughts on them:

So what do you say to people who continue to have debilitating symptoms, despite thorough treatment?
If somebody is working with a therapist and things are getting worse, there’s four areas I always suggest people look at to uncover the barriers that are getting in the way of progress: the therapist, the patient, possibly both of them, or the environment.

Being consistent and compliant with various forms of CBT (ERP and ACT), mindfulness, physical activity, healthy eating, multiple medications, as well as more experimental treatments such as TMS?
I’m assuming that you’re dealing with OCD because you mentioned it later in your comment so I’m going to speak specifically about OCD here: from that list of things you mentioned, many aren’t proven methods for treating OCD. In fact, if somebody isn’t getting expert advice on things like mindfulness, exercise, and healthy eating, I would expect them to simply become new ways of practicing compulsions. Personally, I have no experience with any medications or TMS, but I am familiar with the research, and it wouldn’t be a statistical anomaly if those delivered no results. I’ll talk about ERP and ACT in the next answer…

What do you say to people who don’t even have a chance to relapse, because treatment doesn’t sufficiently alleviate symptoms?
There’s a quote you’ll often see in ACT books that “the solution is the problem”. That’s specifically referring to attempts to “solve” feelings/thoughts/urges that people dislike. Acceptance and Commitment Therapy is all about learning to accept whatever you’re experiencing, recognizing that those experiences aren’t things you have to get rid. The compulsions are the symptoms, not the internal experiences you dislike. In many ways, OCD is all about trying to cope with, check on, or control uncertainty, anxiety, and other feelings we don’t like. The more we engage in compulsions as a reaction to those feelings, the more we experience them. With ERP and ACT, the focus is on cutting out compulsions while experiencing those feelings/thoughts/urges we dislike. So if you’re encountering difficulty eliminating the compulsions, that’s where I’d suggest working with your therapist to figure out ways to make it easier to eliminate compulsions. But, just like with physical fitness, keep in mind that building better mental fitness does involve doing a lot of difficult exercises. Cutting out compulsions through ERP or ACT is almost certainly going to be accompanied by extreme levels of anxiety. It does involve choosing to experience feelings we dislike so that in the future, we experience them less.

What do you say to people who continue to suffer despite continuing to do the things you recommend?
What I always recommend is to cut out coping, checking, and controlling compulsions in reaction to uncertainty, anxiety, and other feelings you dislike, and then to act according to your values to help yourself be healthy and happy over the long-term. Personally, what I like about that approach is that each day, regardless of what’s going on in my head, I can do the things I value. I’ve found it immensely freeing to cut out compulsions and change my relationship with all of the thoughts, images, urges, and feelings that I tried to avoid and control in the past. Learning to accept whatever I’m experiencing as I do the things I value has been life changing. But if people cut out all of their compulsions and change how they relate to the stuff in their heads and they start doing the things they value, but they’re still suffering, then maybe they’ll find a different approach more useful.

All the best as your journey continues.