On XP spending: Mostly, I want the XP we spend to help keep us alive, before spending it on things which just make us more successful. To me, this means we want the things which we are forced to roll (such as awareness) at a high level. Social skills are important, but we're more able to rely on Inoue if we need to, at the very least, that's what I expect we'll do when we get somewhere actually dangerous (socially), or we can just avoid social encounters until we're confident in our ability. This is not true of awareness. Also awareness is BY FAR the most bang for our buck purchase. I don't really see investing one or two points in diplomacy being the difference between life and death in the next five or six updates. I can see one dice of awareness being a difference between life and death in the next couple of updates. This makes me want to heavily prioritize it, even if it means we don't spend 100% of our XP every time. If I thought that one or two dice in diplomacy would be the difference between life and death rather than just relative effectiveness, then I'd agree that we should split it. If you wanted to make sure we spend all our XP, I could see spending it on something like tactical movement, which I think could potentially be the difference between life and death, and doesn't require improving any attributes to improve (we could take it from 2-4 using 5 XP).
On the Black Hunter, after clicking on the explosion release link, I found
this ninja, which honestly concerns me. We're in an area where missing-nin might want to go, in an area with a lot of clay, found a (probably missing-nin) who has an overfondness of and apparent access to explosives. This is a hypothesis we should look for evidence for and against.
On making irrigation: I suspect using a soil water filter may help with the filtering out of monsters - basically, dig a hole near the lake which has water seep into it from the bottom, this water should be relatively free of chakra beasts (then use something else to raise the water into an elevated irrigation ditch). That said, I doubt this would work as a long-term solution (eventually something's likely to get into the filtered water area), and am unsure if it would work at all (this is a quite ambitious project, and there are a number of ways it could fail).