Yes it is possible that CRISPR could work in adults. Scientists are working on it in other diseases. There is a little attention on sz, not a whole lot yet. No guarantee of course, however, I have a couple of studies bookmarked where they gene edited adult mice or rats and alleviated their sz like symptoms. Now it’s hard to say if humans would experience the same degree of improvement, can’t ask the mouse how it feels, and these studies have all been on different genes that not everyone has. Different genes express at different times, so there’s obviously going to be some water under the bridge as far as brain development, but many genes continue to express. If they are expressing too much or too little of a protien, or coding improperly, it’s a plausible target.
Any universal stem cells will be carefully tested to make sure they only produce the desired cells and not cause cancer, but they’ve been doing cell transplants with some success in Parkinson’s for years without ill effect, and they haven’t had the technology they do now. They also haven’t been able to get enough dopamine cells to survive to get a “cure” in Parkinson’s, but they’ve improved and legnthened people’s lives. Also, since we’re inevitably talking about a therapy that won’t be available for many years, by that time I suspect that they’ll be diagnosing cancer with routine blood tests and sending in gene edited bacteria to kill it while it’s 1mm wide or something like that.