What is the difference between types of propeller?
There should be two parameters you should know about:


Blade loading: The amount of force per unit of area a blade is creating. Like with a wing, you can formulate a lift coefficient, a dimensionless number which removes the influence of size, air density and speed. Ideally, this number should be between 0.5 and 0.7, so the propeller makes best use of its blade area.
Advance ratio, which is the ratio between the circumference of the propeller disc and the height it would screw itself up into the air in one rotation. Increasing the blade angle would directly increase this advance ratio, for example.


Now you should be able to guess the answers to your questions:


Adding more propellers: Will distribute the load over more propellers and helps if blade loading was too high, but makes matters worse if blade loading was low already.
Adding more blades: See 1.
Replacing blades with wider blades: See 1.
Making the angle of all blades steeper: If you run the prop at a lower speed, your blade loading will go up and thrust will be the same (if you don't mess up the flow over the blades by changing the angle). If you run the prop at a higher speed, blade loading will go up; again until the flow cannot cope with the angle anymore. This is similar to stalling a wing. Changing the blade angle is the preferred means of adjusting the prop to different flight speeds in airplanes.
Adding a second, contra-rotating prop is like adding another sets of props (see 1), but helps to reduce swirl losses. If blade loading was at a proper value before, halving blade loading will create more losses than reducing swirl losses. You add contra-rotating props only if your blade loading was high already.
Spinning propellers faster will create more thrust but also more losses. If your thrust is not sufficient, it is better to increase prop diameter (a parameter you forgot to include).