Great review. I almost couldn't believe it when I first found out #25 was ghost written, it's a solid book. Admittedly I'm biased towards liking Marco books, but I still really enjoyed this one. If all the ghost written books had been as well written as this one, I doubt many people would've realised. Same goes for the following book #26, one of my favourites.
I suspect Visser Three's torture device collection is just an added attempt at intimidation on his part, although this is made rather redundant considering his reputation for impatiently decapitating anyone who even slightly annoys him.
I actually forgot the part where it specifically says that the Venber are not carbon-based (I believe they are thought to be hydrogen-based), but I'm pretty sure that makes a cross breed with humans completely impossible.