Hi! Okay, so, you’re addressing a lot of things at the same time, I’ll try to answer everything. 🙂
First of all, I haven’t seen Akame ga Kill, so I can’t speak for this series but as for the rest…
Yes, you read right, this is exactly what happened. As I said before, the act in itself was foreshadowed once but not in its… settings and the sea of adults that were waiting for them, so this was an additional detail that made everything worse in my case. 
Sensei was not particularly graphic, but enough so that it’s not hard to decipher exactly how the twins were… used.
I think it’s great you managed to avoid the spoilers even if the chapter in itself must have been hard to swallow afterwards. Personally, I needed a few days to feel better after reading ch135.
Fair warning in advance, but ch136 will probably contain other triggering scenes about child abuse. I listed all of the moments from older flashbacks that we have yet to see here, I hope it will be useful to help prepare yourself for January’s chapter. :3
I understand that you watched the anime of 1997? If so, then yes, they really toned it down compared to the manga.
I’ll be honest here, Berserk is one of the most amazing manga series I had the chance to read, but it is very explicit and violent, especially in its first half:
- people get violently butchered and tortured
- there are different kinds of rape scenes
- just like there are rather explicit sex scenes
- a lot of child abuse
- pedophilia and incest are less recurring but they’re addressed at least once
To answer your other questions, forgive me if I spoil something for you but yes, Guts gets raped as a child, only once though, because his adoptive father literally sold his ass to one of his comrades. Guts actually gets through a lot of abuse as a child in general and his name is the first clue you get, since it’s a sick joke about the circumstances of his birth.
As for Casca, poor girl, many men attempt to rape her throughout the story, unfortunately for her, because women in Berserk generally aren’t fighters, which makes her a big exception and the only woman constantly surrounded by angry and violent men so… yeah. Even Guts, who can’t stand rape ever since what happened to him, half-crossed the line.
Still, it happens only once for her too, but it’s with the most traumatizing circumstances possible: they’re literally in Hell when it happens, her comrades were just butchered in front of her, she’s held down by several Apostles (demons) while her “friend” decides to do his business right in the middle of the massacre, as a punishment for both Guts (being forced to watch) and her.
So yeah, it’s definitely not a moment I go and reread often because Miura-sensei made sure to make it hard to stomach from beginnning to end.
I really dislike the anime, but the third movie shows this part of the story and… I think it was “well done”, because it takes everything in consideration and more importantly, you can hear how angry and desperate Guts is.
Finally you asked whether you should read it or not, but I really can’t answer that.
It has a lot of triggering content, so from the very first chapter onward, you can see what you signed up for but this doesn’t mean you will get used to it.
My best advice would be try and see. :)) With what I described you should know enough to be prepared and besides, you’re allowed to take long breaks while reading, just like you’re allowed to just stop if you can’t handle it.
I won’t lie, Casca’s rape and Kuro ch135 were hard for me to stomach the first time I read through these scenes, but after a few days it gets better and you don’t need to go back to these moments again anyway.
So it’s up to you: Berserk is amazing enough to deserve a try but I also know that not everyone can handle how violently graphic it is, so no pressure whatever you decide. :3
I hope it answers your question, have a nice day Anon!