SLIGHTLY LONG POST BUt here’s my thoughts (mind you I haven’t read much more chaps than you have);
1. I don’t think the servants know, based on yoshie’s behaviour…
2. We guess Kenta is around 5 years old. Ofc, there’s nothing sexual about a five year old, nor is there any sexual interest. But I understand that you probably mean trope-wise… Manga has a sad history of sexualizing children and siblings as a commonplace type of thing so it’s understandable to believe that’s where it’s going for the sake of tropes. I hope Nojiko stands above that but ya’kno… BL…
As for his behaviour, I think it’s really important to remember that Kenta has had a terrible childhood. The only glimpse we got was some sort of cage (unless that was Masato). He’s suddenly put in with this rich family, where his brother is the only one “there” for him (yoshie is more like a parent and making him do things he doesn’t want to do, masato plays with him).
A childs brain work differently to adults. They’re unhindered in their thoughts and speech, they often say fleeting thoughts that come to mind and explain things in very clear cut ways. It sounds disturbing to an adult, but as a child he’s simply explaining things in the best way he can think of. From what he’s been saying so far it’s obvious he’s terrified of going back to the place he was at before he got “saved”. Regarding the last line in ch2, it’s perfectly normal for children to say something (unhindered), reflect upon it, realize it’s not good and then take it back like he did. So far I think he’s displaying kiddie behaviour, but of course it’s the kind that would make a childs parents worry (since he’s obviously very traumatized and scared, thus developing the very bad kind of dependancy).
3. Can’t tell you if the dickbags backstory will be revealed, but what we know so far is that he’s filthy rich and travels for work.
Hope that helped ya!