I just read this, and liked it. Bit of a ride, but enjoyable, and the ending is fine. The comments are almost better, though.
Why would Ruki forgive Sacchi and even Remi? Well, it was presented as Sacchi discovering (via sex with Remi) that she could like having sex with girls, and then discovering that she actually did want to be with Ruki. If the cheating hadn't happened, they might have simply broken up. So, works out for Ruki.
Remi literally tried to use powdered drugs on people,
According to Maasa, it was wheat flour. (chapter 3) Remi was apparently hoping to lower their inhibitions by making them think they'd been drugged. Like the trope of characters acting drunk on grape juice or non-alcoholic wine.
Remi's kind of creepy but it was Asuna's rape of Meru's persecutor that really raised my eyebrows as a story element.