Caption: A schematic diagram (called a free body diagram physics jargon showing the forces acting block on an wedge (or inclined plane): the forces being gravity W downward, the normal force N perpendicular to the contact surface, and friction F parallel to and up the incline at the contact surface.
Note the forces that act on the block NOT the forces the block exerts on its environment according to Newton's 3rd law of motion. If you try to include those forces, you create confusion with seemingly canceling forces.
Features:
In a physics description, the 3 forces can add as vectors to zero, but without friction the 2 remaining forces CANNOT.
In fact, every bit of the block and wedge would slide and both would spread out into infinitely thin film on the horizontal surface.
Real fluids have some viscosity and also surface tension (which is a force resisting spreading out), and so you don't get a an infinitely thin film when you try to build a hill of fluid, just a very thin film.
A pair of shearing forces are parallel by do NOT act along the same line. Thus, they tend to make layers of a body slide over each other.
This is just what happens in fluids.
Only the pressure force is strong enough to resist sufficiently strong self-gravity. Atoms strongly resist being compressed.
But note the pressure force does NOT resist shearing forces.
So when the pressure force and gravity balance on each microscopic bit of matter flow can stop.
Gravity acts only toward the center.
Each spherical shell has uniform pressure. So a radial pressure gradient balances gravity at every point radially. And there is NO tangential pressure gradient to cause a net tangential force.
Well this depends on chemical composition, heat energy content, and rotation.
However, observations suggest the empirical rule the size scale for a rocky astro-body must be >∼ 600 km and for a water ice astro-body must be >∼ 300 km (see Wikipedia: Dwarf planet: Hydrostatic equilibrium).
General relativity (GR) predicts yes. A sufficiently compact massive object will collapse to being black hole. However, the interal structure of black holes is very uncertain.