Hey so lots of questions!
I'll answer these in a generic sense rather than maya specific for the most part, i'll mention a few approaches used to achive the kind of effects you are after and why as well.

1. Leaves blowing around.
Approach one, simple particles:
So for many cases if you have a quick shot where there isn't much complex motion e.g. eddies.
This can be achived by doing a simple particle simulation with some friction and animated forces.

Simple particles are generally dumb, so each particle has little to no knowledge of other particles , you can do particle - particle collisions but these will be simple implicit sphere collisions where the solver will just move a particle out of other particles at each step. Particles in most applications do poly collisions so you can get collisions/ friction/ bounce with objects in a scene.

Instancing and collisions:
With particles like this there is no geometry representation in the solver, the geometry you instance on to the particles has no impact on the simulation. Any collisions are done by a radius on each particle.

Orientation:
Simple particles usually have no or a very basic concept of orientation, with nParticles the integrator can handle rotations but they will not be good for things like leaves but fine for hundreds of pieces of rubble.

Orientation can be computed post simulation by expressions based on the change in direction of velocity.
e.g. the angle difference between the last step and the current step based on velocity can be described by acos(dot(normalised(previousVelocity) , normalised(currentVelocity)))

Approach two, fancier simple particles:
One thing that is frequently done to enhance the look of a particle sim for leaves or particles larger simulations is to advect particles with a fluid simulation. If you did a simulation of some smoke blowing through a corridor you can use this to move the particles though the environment, essentially replacing other forces you might use.


Because fluid simulations can capture vorticies (eddies ) this will give you that incompressable volume motion you're looking for.

A note on particle instancing:
With particles you attach a geoemtry to each particle and render time ( instancing ) by default this is usually a static geometry,however you can cheat the feel of bending by attaching a sequence of geometries that can shift to give the feeling of you're geometry changing even though its no simulated.
This could be driven by the velocity of the particle for example so you could have the leaf bend more as it moves faster.

Approach Two, cloth:
This is probably the most straight forward way to get alot of what you are after, if you represent each leaf with a simple plane with a few subdivisions and convert them into cloth objects, you will get the look of the leaves bending with the motion as well as everything else you get from particles. However I don't think you can simply advect this by fluids in maya though this is trivial in other applications.

Approach Three, rigid bodies:
Ultimately this will give you a similar physical motion to particles but the collisions are resolved properly with geometry so you will get a much better integration. Obviously by definition these are rigid solves but it is possible to give the feeling of bending by building a leaf that is made up of multiple rigid bodies that then drives you're original mesh.

For example you can scatter a few spheres over a leaf ( or convex decompose the leaf if you can) and then use these as collision geometry in a rigid body sim and create constraints between each body to give a feelign of bend in the simulation.

A wrap or lattice deformer could then be used to deform the orginal leaf geometry by the rigid simulation.

This is NOT EASY in maya though more trivial in other packages like houdini.


2.
The spear example generally wouldn't be a simulation but would be hand animated.
Rigid body simulations generally don't handle this kind of situation.
If wood splintering was needed then either an finite element solve or a rigid body solve would be used to break the shield and then secondary particle simulations would add detail.

A finite element solve is only different from a traditional rigid body solve in that the constraints between a single object such as a piece of wood are solved all at once rather than individually which allows for a sense of better energy propergation through a matterial. This is usually coupled with dynamic fracturing which makes life easier in some cases. ( though really isn't as magical as everyone thinks it is).

3. 2 FPS is totally expected for a large rigidi body simulation, you should be caching the simulation to disk before rendering. You're workflow should be:
1. work on youre simulation
2. cache you're simulation
3. render the cached simulation.

Simulation time should not effect render time at all.