Hello, Although it's said that "about" means ~에 대해서 or 관해서 in Korean, which is right in most sentences, but I don't feel 고구마 먹는 것에 대해서 말하고 있었어 sound native or natural to me. It does depends on the situation but I would say 고구마 먹는 이야기 하고 있었어. I'm sorry I can't completly expain it.

And about 체험한 이야기. Because it's like the name of a book it can mean different from what I wrote above there. It really depends on. From the view of a native speaker of Korean, even I can not be 100% sure that I understand the meaning of it.

여섯 살적에 나는 《체험한 이야기》라는 제목 의, 원시림에 관한 책에서 기막힌 그림 하나를 본 적이 있다." Ok, it's pretty much challenging question haha. For starter, 적 in 6살적에 means around or about 6 years old and it's not really a formal way to speak or old people can use that while speaking. Second 적 in 본 적이. 적 is usually reffering to experience that you've done before. Yes it is just like the grammar "Have p.p." In English. However in this sentence, it just makes it clear that it happens in the past and it can imply that you're going to talk about further details on it. Like, 예전에 한국 음악을 들어 본적이 있어. 그런데 내 취향은 아니었어. Ok I'll try translating it! When I was around 6 years old, I once saw a spectacular painting from a book on primeval forest called ' The stories from the experience'. I hope i'm able to help you out with this.

저는 영어로 번역된 공주 이야기의 초안을 수록한 파일을 발송했습니다. would be more natural.

녹이다 is a transitive verb and 녹아내리다 is an intransitive verb! I also apply to 녹여 and 녹아내려. 녹이다 for 녹여. 녹아내리다 for 녹아내려!