@Gwenfar I did finish it, and I think I'm glad I bothered, but the flaws definitely do continue, and there are things I'm still confused by at the end of the book.
Very quick summary (I'm not sure exactly where you left off, so sorry if some of this is repetitive): They start exploring Rohrshach, and gradually figure out that it's a complex lifeform itself. It makes very clear from the outset that it doesn't want them there, and basically mounts an immune response, but they're able to make repeated trips. In one of those trips Szpindel is killed, and so they reanimate the backup biologist, who is a complete dick and who I doubly hate because I found Szpindel by far the most likeable character.
Anyway, inside Rohrshach they find these very agile many-limbed creatures that they call "scramblers", and they capture a few. The first one doesn't survive, the next 2 they keep as experimental subjects and make them do increasingly complicated problem-solving to try to learn about their abilities and communication. At first the backup biologist insists that they can't possibly be intelligent because they don't have brains, but they massively outperform humans on a whole range of pattern matching and inference tasks so it becomes clear that they must be.
Eventually the crew pieces together a notion of Rohrshach as spectacularly intelligent without consciousness, which is also why talking to it felt like talking to a Chinese Room. And the scramblers are individually intelligent but don't have any sort of self concept, and they realise that they only got away with capturing those two alive because Rohrshach allowed them to be captured as spies. The captive scramblers also start to get weak because they depend on Rohrshach's magnetic field for some biological regulation.
There's a lot of arguing about what it means to be intelligent without consciousness, including a line of argument which I'm not at all convinced by in real life but made sense in the fictional context: that consciousness is actually an evolutionary drawback because conscious thinking is so much slower than unconscious decision-making.
Then two things that I'm still confused by.
1) I'm not sure what the actual order of events is here, but ultimately Theseus gets closer to Rohrshach, Rohrshach starts actively attacking the ship, and the crew except Keeton all agree to fly into the thing as a massive antimatter suicide bomb.
2) Sarasti gravely injures Keeton to make some point about consciousness. I didn't follow why. There also seems to be an implication that maybe vampires aren't really conscious, but that didn't quite make sense to me.
The book ends with Keeton in an escape shuttle on his way back to Earth, his mission now being to bring the whole account of what happened back with him. It's implied that the suicide mission worked so Rohrshach is gone, but it's also implied that meanwhile on Earth the vampires have rebelled and slaughtered humanity, so Keeton might be the last living human.