I have told a few friends a few things very slowly, but only when they had a helpful response to information that wasn't the most sensitive for me. Family... I told a few things to one who has medical training and responded well to less crucial stuff, similarly. Most close family, though, were either abusers or deniers as I was growing up, and some died years ago.
People all have their own struggles. I've had people start to react as if I'm trying to be the person getting the support and never giving back just after I said a simple small thing about abuse. It doesn't really feel fair that they do that; maybe they find the topic overwhelming, or they are scared that I will ask too much... but I've found the people who are capable of being supportive to be pretty rare.
I do really try to give back to the relationship in the few cases where it's become safe to talk a bit. Therapists are best for me for that, though, no worry about having to take care of their needs if I'm having a really bad period, like some friends/family.
Not sure why, but I have done (b) and (c) more than (a).
(c) People can go to therapists without having mental health problems.
(b) Trauma can result in lots of things other than ptsd, and I think people can be quite empathic on things that don't trigger our favorite thing -- victim-blaming -- easily. I've found people quite supportive re one traumatic experience I couldn't hide -- being robbed at gunpoint near work... Although that really resulted in few problems for me compared with the childhood stuff, people took it very seriously which surprised me. Ok I'm weird but my family were experts in minimizing abuse, incl. death threats, for years, so how am I supposed to respond... It has helped me feel safer around people in general about their responses to trauma that they will empathize with me for certain things anyhow.
(a) The mental health problems one, though, I have done my best to avoid disclosing! What is ptsd anyhow, the official line changes every year; is it an injury or an illness, would all people get ptsd if subjected to enough trauma, what is going on in the brain of people with complex childhood trauma, how much can it be healed -- all these things seem in flux... Different people believe different things about different types of ptsd and even different traumas! "Should" childhood abuse result in this problem or that problem... and then there is the very serious issue that abusers are out there too and often have motivations for having strongly invalidating reactions toward survivors.
I don't feel safe disclosing, anyhow, a lot of the time. Some people feel it is a political issue to disclose I guess. Others feel safe -- some have had enough support that they can more easily deal with negative reactions, perhaps?