Having skimmed through this thread, I must say that you have some very excellent ideas.
Volume of rubble could be suitably combined with the sand/semifluid flow mechanics and machinery to produce, say, a water-wheel-driven (or capstan-driven - a use for livestock! But I digress) conveyor belt system. Mining dwarves dump the detritus onto this belt, and it is carted away, eventually dumped over an ever-growing pile of pebbles. The lowest end of the conveyor belt becomes a dump zone for rock debris; within that mine shaft, anyone set to clear away the debris will take it to that conveyor belt, and can load so much onto it depending on how fast the conveyor is turning(and maybe how well it's made). They basically become a temporary semifluid source.
Speaking of fluid, though, your notion of a "dangerous fluid level" flag caught my attention as being difficult to work with. How do you define a "dangerous" level? Do you go by what would be an impediment to a human? A dwarf? A dwarven child, a baby, a cat, a roach? Although I would love to see fluids implemented as you're saying, I don't think the pathfinding angle could possibly be settled so easily. It makes little sense for a full-grown dwarf to be stymied by water that comes up to his knees - but it makes little sense for a cat to traverse that same level of water without needing to swim.
That said, it might become moot if the notion of fluid level gets interpreted more intelligently. At its most abstract, you could say that any water up to a quarter of someone's height is OK; between a quarter and 1/2, they are wading and will be impeded somewhat; between 1/2 and full height they can wade or swim; anything over full height and they must swim. Some creatures will be significantly faster swimming than wading, and some might even be faster at swimming than walking(while still able to walk perfectly well - contrast fish), but wading will generally be the slowest mode of progress for any such creature. Yes, strictly speaking you have to swim when the water is up to your mouth, but as it stands the raws don't have enough detail to say exactly where your breathing-point is, and I'm not sure it would be necessary. You could set the trigger to 9/10 of the creature's height, I suppose, or do a bit more fiddling there. Maximum/"normal" stance height, and the proportion of that which is the minimum, become important to know. Maybe different modes of motion could have different values, and be independently definable. Below your normal stance height, you will need to stoop. Get pushed down further, and you will need to crawl on your hands and knees. Further still, you will need to crawl flat - and if you can't do that, the tile is impassible. Four-footed creatures wouldn't have as many modes, though, because they don't walk around on their hind legs(not as a means of travelling any significant distance). They can walk, or they can hunker down somewhat, or they can flatten out and crawl, but they don't have the intermediate crawling stage that bipeds do, because that's their natural stance already.
Creatures who are less dense than normal would start swimming at lower thresholds, and with reduced consequences for swimming failure. A water bird is not likely to have a problem staying afloat even while young and clumsy - they just float too well to be imperilled unless they do something really bad(hit their heads and pass out, say, or get tangled on something and dragged under). Reduce the thresholds according to their relative density to water. A typical mammal is very near a density of 1.00 - slightly under it, but left to physics, very little of them will be above the surface. Increased thresholds won't affect the point at which creatures start wading, but will change when they can start swimming. A warrior in full plate is not going to be able to swim at all, which means that if in water over his head, no amount of swimming skill will save him - he might be able to jump around a little to stave off the inevitable(assuming he's not in much deeper than that), but if he doesn't get out of deep water, he will drown.
Currents and especially undertows would complicate matters, of course.