Ah, don't mention it. Trust me, the physics of gryoscopes aren't exactly straightforward, but i'll try the best i can.
Your first question: Well, consider a spinning top. If you observe the top axis of the top (excuse the pun), notice that it moves in a relatively slow-moving circle, the speed of which the top axis moves is the angular speed of precession. The actual speed of the spinning top is the angular speed of the top.
In the case of the bicycle wheel on the axle, the axle will actually rotate about the pivot (at a relatively slow speed, this is the precession speed), when the bicycle wheel is rotating about the axle (this is the angular speed of the wheel).
2nd Question: The angular speed of the wheel can be found by placing a mark on the wheel, and timing the time it takes to complete, say 10 revolutions, and taking the average period from there.
3rd Question: I = mr^2 is an approximation, assuming that the wheel can be modeled as a hoop without the spokes. To vary I, you need to vary the size of the wheel you use, and this affects the weight too. So, these two variables must be varied together.\
PS: If you need a comphensive explanation on how gyroscopes work, feel free to ask.