When my main abuser, my mother, had died and I cleaned out her house, I found letters from her to some family members. She did not (!) actually write them for them, but for herself, which was part of an attempt to heal her cancer by doing soul work (long story... she had fallen for a so-called "spiritual healer" who is known throughout the world for a fake, etc.). Anyway, there they were and one was addressed to me.
Yes, it was an apology and it showed very clearly that she indeed had the capacity of empathy and compassion, but said herself that she had to become terminally ill with cancer to see what she had done to me. The letter is about 5 pages long and contains some things she had come to see finally.
Her "seeing me" and expressing that did a lot of good for me and still does when I read the letter. But, what it cannot do and will not do ever is change the relationship we had; she hasn't become more of a mother to me, not even in my imagination, plus it is one letter, and not even written with me in mind, but with herself in mind. I do see that as well. I do think, from the way she wrote, that it was partly for me, but I do not know this. It was part of a notebook which included "spiritual" work to heal from her cancer. I found the "spiritual healer's" "workbook", the letters were corresponding to.
I was abused by that woman every single day of my life until 19 (when I moved out) and 99% of the time after that whenever we communicated. Only in the last few years of her life did it get better, which had to do with her becoming aware her own abusive history (we talked about this once).
My cousin, whose mother (my mother's sister) abused her also every day of her life, until this day when they communicate) envied me very, very much. It depressed her greatly that I had gotten this from my mother. At least one letter, she said. At least you know she has come to some understanding. Her own mother has been drunk, on drugs and on prescription medication as long as I can remember back, and I'm the older one of my cousin and I. Her mother couldn't even see those things anymore because she has killed many of her brain cells with all the substance abuse. My cousin was aware of this.
I am saying all this because I do understand my cousin's view and that of others who didn't get such a letter/apology. But it is not correct. "Some understanding" does not make up for any of the abuse. It does not heal you. It does not make the PTSD any better/easier. It does not change your relationship necessarily. One letter is what it is, one letter. So, this is more than nothing, I agree, but not a lot more. Does it sometimes pick me up? Yes. Does it put me back down again when I see her later work (corrections, changes with regard to workbook questions) on top of the original letter in different color? Yes. Was it for me? No. Not primarily. Did she take the opportunity when she was still alive? No. Did she give me a chance to react to it? No.
Of course, this goes for my letter and my situation.
There is no general answer to your questions. It seems to me that my mother wrote that letter with love for me. That does reach me and touch me. But there are the other truths as well, as I mentioned. What would have helped me would have been facing me with that load she put upon me and actively dealing with it. Meaning, writing that letter to me when there was still the possibility to communicate, to react, to share, to see her and get my own impression of how much she had come to understand of how much it was with me in mind, and changing our relationship, not into a mother-daughter one, it was too late already by then. But to make it better, to arrive in the present in some way good for both.
Still, I am grateful for that one letter I got. I found when I read it first that it gave me one root (thinking of children being a tree needing roots to be able to grow strong) that found its way into the earth. One root is not enough though, but it is one root and therefore not nothing.