Hi there T!
Beautiful building site! And good drawing, too—I’ll try to answer
your question as though you were building in the U.S., knowing that our codes aren’t applicable in Portugal. Still, you might find some of it useful.
I won’t get into whether such a tall foundation made of (presumably mortared) slate is adequately reinforced for what will be a very heavy plastered straw bale wall and roof assembly. Where I build in Oregon this might be considered a top-heavy structure, and with earthquake activity a possibility would require a lot of reinforcement since straw bale walls are five times heavier than conventionally framed, insulated, and sheathed buildings. Once things start moving in a seismic event that’s a lot of weight to restrain.
RE question 1. The metal weep screed at the wall base is a good idea for a couple of reasons. It gives you an edge to gauge plaster depth, gives the plaster a place to stop, protects the plaster edge, and if you drop it another inch or so below the joint where the sill plate meets the foundation wall, also protects against wind-driven rain entering the wall while still leaving an opening for water to drain out (heaven forfend that water should ever get in, but in case it does, it’s a precaution taken on structures I have worked on).
The weep screed and plaster don’t need to rest on a ledge or shelf in the foundation so long as it’s not playing a structural role—most conventional buildings in North America that have +1” thick stucco (render) exterior finishes don’t rest the plaster on a shelf cast into the footing or stem wall. Even though the plaster alone weighs 15 lbs. per square foot at 1” thick (150 lbs. for a 10’ column!), the lath-mesh stapled to sheathing holds it in place, and the same is true for the lath supplied by the rough straw bale surface. You may wish to add mesh for out-of-plane force resistance, or employ some other method like paired external pins, but you don’t need it to give the plaster enough tooth to hang on.
If you’re relying on the plaster skins to supply resistance for in-plane shear, and you haven’t engaged an engineer to calculate whether the staples holding the mesh in place will be enough, then the prescriptive path outlined in our building code requires the plaster to rest on a shelf cast in the footing or some other engineered design. This is true for both load-bearing and in-fill designs. Resting the bottom edge of the plaster on the foundation gives it something to bear
against. Seismic tests conducted in the U.S. showed that the test wall fared better when the reinforcing mesh was stapled to the sill plates on a specified schedule and the plaster could bear on a solid surface as the applied lateral force (simulating an earthquake) attempted to turn the rectangular wall into a parallelogram.
RE question 2. Here in the U.S. we need to place a waterproof barrier between concrete foundations and wood sill plates, even when the sill plates are made with wood treated to resist decay. That’s probably not a bad thing if it makes allows buildings to last longer. I’m not sure what our code says about sill plates over stone. Whether stone allows moisture to rise through it like concrete I can’t say, and given the tall slate stone wall you have I doubt it would rise that far via capillarity. Still, I’d take the precaution of using a waterproof material under both sill plates and the space between—whatever is commonly used in your country. In my practice we use a flexible peel-and-stick membrane that we run up the side of the interior sill plate (after that sill plate is secured to the footing with a layer of some waterproof membrane under it), then run that membrane out to the edge of the footing. The exterior sill plate sits on top of it. That way if water ever drained into the area between the sills it would flow to the exterior. This method doubles as an air barrier so long as the interior plaster drops below the top edge of the interior sill. Some builders cut narrow (1/8” x ¼”) drain channels with a saw every few feet in the bottom of the exterior sill—not a bad idea if you have a tight seal there—but my experience
is that the somewhat rough surface of the foundation is uneven enough that water drains.
Scott's detail showing a "Z" flashing over a weep screed or casing bead above a window is one that I use as well. It directs water flowing off the wall above away from behind the window. One difference I'll point out is that if you are trimming windows with wood I'd continue the casing bead (also called "J" channel) around the entire window. That way when the wood trim wears out (gets destroyed by sun and water) it can be replaced without damaging plaster that might have stuck to it.
Many Hands Builders