Great thread eeks! This is such a valuable topic.
The current reality of mental health services is that people who would like to improve their mental and emotional well-being are often left to do "trial and error" on their own, to find modalities, therapists and other health practitioners who "work for them", that is, result in a shift in symptoms and increase in self-awareness and/or well-being.
I saw one therapist, once, prior to finding my T. They were in the same office but the first one did not fit and I am so glad I called back and asked if there was someone else I could see. By that time I knew about BPD and these boards, I started here on the Leaving board having an exBPDgf. I specifically asked for someone who treated pwBPD, I assumed this would be the ticket for someone who understood everything I had just been through. (Though looking back now, it wasn't the first r/s with a pwBPD I had been in.) I hit the jackpot with my T.

there are countless books, websites, programs, produced for the purposes of "self-help" by people who aren't qualified as therapists, and yet claim to be able to help people. How do we evaluate their claims for ourselves?
I've gone with the motto if it helps, read it. Take what works and leave the rest. I think a balance can be found if one doesn't become a guru groupie. I think we can better evaluate someone's claims to be helpful by reading credible material first. Using these boards and working with a T also helps with that discernment.
Many of us using the discussion boards on bpdfamily.com have a mental health diagnosis of our own, and it is my opinion that being in emotional pain makes a person uniquely vulnerable. Not weak, not lacking intelligence, but vulnerable.
I have a PTSD diagnosis. I agree with your assessment, though I do think it's possible to become more comfortable with that vulnerability.
all I knew was that I'd heard therapy is supposed to help with emotional stuff. So it becomes even more important to have reliable, informed and responsible mental health practitioners.
I couldn't agree more. Years before my last r/s I dealt with my brother's addicts and his entering recovery for the first time. I was fortunate enough to be hired on at a recovery facility, I did remodeling work there. (Not the same one he was in) I had access to all these "professionals" who were knowledgeable in recovery. I would sit in on some of their group sessions just to learn. I had a lot of questions about my brother's addictions and how I should be handling things. That's where I learned the term codependent and that I had codependent traits. I had no idea however, what that truly meant and the depth.
Fast forward to finding these boards and my T, I had no idea back then what an enormous difference between having a professional T of my own and having occasional chats with alcohol and drug counselors would be.
Practitioners vary widely in their ability and willingness to, for example, tell you their services are not the best thing for you (or they're not qualified to treat mental health issues) and refer you elsewhere.
how do you evaluate mental health information, and the claims made by practitioners? What factors tell you someone is trustworthy? What are the red flags?
Sad but true. One has to investigate as thoroughly as possible. Even if someone is licensed, check their history, ask a lot of questions. A professional that has a real license at least has to be held to certain standards and ethics. Red flags? Anyone who would give a diagnosis without spending a substantial amount of time in sessions with you would be the first for me. Making promises to you or doesn't exhibit good boundaries of their own, for example sharing too much about their personal life with you.
But some people resist the idea of "spending all that time digging through the past" and so a quick solution appeals to them, and to be fair, some people may not need to explore their past. How do you discern for yourself how deep you need to go?
For me, I found the more I dug into why I've been in dysfunctional relationships most of my life the more I needed to look into my FOO. It's been like the old saying "recovery is like peeling an onion", there are many layers.