Now that I've finally read both books, it's interrogation time!
#1: Some leeches look vastly more powerful than others. Is this determined by the availability of their wells alone (e.g. "sunlight" leech would have access to much more power during the day than a "gold" leech), or do they also have individual power levels?
Sorry I missed this question, Evgeni -- here's the quick and dirty answer:
A leach's well is on available when the leach is in proximity to said well. So, if a leach relies on seawater, there's no power when that leach isn't near seawater. Like gravity, the well's power decreases exponentially with distance. Take the following equation as suggestive rather than precise, but I imagine a leach's power (P) to work something like this: P= (I x qW)/dW^2. In this equation, I= innate ability of the leach, qW= the quantity of the well (how much sea water is present), dW= the distance of the well from the leach.