1: Demon-frog-vampyre-things: Very good. The problem is, I can't see them evolving from anything mammalian or avian, ruling out warm-blood.... Unless.... What about if they had primate/Australopithecine type features? Say, for instance, a primate analogue, something like
swamp monkey in terms of size and appearance, though of a dark red colouration and with small, stubby horns. They would also be nocturnal.
When the Dark Fen flooded, these 'monkeys' grew larger so they could go down to the flooded forest floor and remain with their heads above water, developing a bipedal gait and slightly webbed appendages in the process. This semi-aquatic lifestyle would also lead to losing their fur. The eyes would be cat-like.
This leaves us with animals of chimpanzee intelligence, no fur, horns, deep red skin, a prehensile tail and a bipedal gait, plus webbed feet developed to help with swimming. I'd think about five foot tall? But hunched slightly as well. I must get LD to draw one. I think they could be very creepy. I think I also like the face being very gorilla-like, with perhaps slightly elongated jaws. They are predatory after all. They also need claws I think.
2: Nessie: Well, I'm not sure. I did not plan on having marine reptiles so far north. I would expect them to remain in more southerly, warmer waters. A pliosaur equivalent, something like Liopleurodon, would be one of the apex predators in the Midland oceans, but not too far north. To tell the truth, I like the idea of oceans populated by creatures other than fish. But I'll have to get back on that one.
3: Adrenal Necrosis: Sounds pretty damned awesome actually. I can see I'm going to have to really work out how it's going to work mechanically.

4: Invarrians and Death: Invarrians are just incapable of taking things seriously.

No, that's not entirely true, but they live in the moment, taking all they can from it. Their nonchalance towards death stems from an astounding racial mix of pragmatism and optimism. Like most dogs, they're just really playful. So their view of everything being a game is a direct result of their optimism, while they realise that, for instance, death, is something everyone must go through. Who knows? It might even be fun. Getting to this point has been, death might be too.
5: Armadillos: Assuming you mean actual armadillos and not excrement

, I have given thought in the sense that there is a large glyptodont type creature living on the Boltmoors. Beyond that though, no. Although an armadillo-people might be pretty interesting. Natural armour and the like. If I can think of an interesting culture for them, I might just have to pinch it.
Cheers both of you,