I don't think it's necessary to have rules/agreements on how to roleplay each and every gameplay element. I'd rather trust our imagination and just make the details up on the fly, rather than try to memorize a ridig set of rules. Apart from the harvesting problem, which I know nothing about, most of those mechanics/roleplay translations are pretty straightforward.
Nevertheless, here's my input on the topic:
Hit points: quite simply, they measure how close your character is to death. Usually the lack of hitpoints means wounds, sometimes another kind of health hazard (like goo damage). If we want to really go deep here, I'd say that only the wound that knocks you out is truly serious. After all, you can go on normally if your health is at 1 and it naturally regenerates all the way back to full.
Stamina: physical exhaustion from a long hectic battle or whatever best suits the situation.
Focus: a little trickier, but I suppose it measures your ability to focus intensely on your task. When it runs out you get disoriented, because your mind just needs a break from all the thinking/looking for open spots in your enemy's tactics/whatever. Whatever you were doing, you lose focus of that.
Sap: not sure if it's explained differently somewhere, but I'd see this not as actual physical sap slowly dripping from a treeside. (If it were, how would you regenerate such sap during battle, and why would you have a maximum based on your mental stats?) Rather, I see it as a mythical life force, the blood of the planet that flows within us all and can be drawn on to perform extraordinary deeds (and it just happens to be called "sap" instead of, say, "mana").
Remember that this kind of set of four numbers to describe the state of a human being, is always an abstraction, and the same mechanical situation can be roleplayed in countless equally valid ways.
As for the metric system, it would seem likely the Karavan kept that system around when the rest of the world went medieval.