What are the chances, if the motor propeller/motor shaft turned fast enough, that this would reliably power an Arduino Uno?
If the motor spins "fast enough" then it would by definition be able to "reliably power" something, otherwise it wouldn't be spinning "fast enough", now would it?
The question is how much power this thing can generate at a given airflow.
It claims 0.01-0.3A output current, but under which conditions?
You'd have to ask the seller for data or buy it and test it yourself.
More Questions
1. Is it correct that the baseline load presented by an Arduino Uno (with nothing much going on other than lighting the onboard LEDs) is about 100 Ohms?
Sort of, it idles around 45 mA.
2. What happens if the DC generator's RPMs vary and the resulting voltage from the generator varies, and therefore the Uno sees more or less voltage (for example 6 Volts or 4 Volts), and therefore draws more or less current?
There is a voltage regulator between the generator and the load.
As long as the generator stays within the regulator's specified input range it would provide 5 volts.
If the input voltage becomes too low or too high then the regulator may stop proper regulation or shut off.
3. If the RPM (and therefore the voltage) of the generator is likely to vary, would it be best to power the Arduino via the USB port, or the DC power jack, or the VIN & GND pins. Which would provide the best Over Voltage/Over Current protection, and voltage/current regulation (if any)?
The DC in port goes to a regular which turns whatever comes in into 5V.
Since you already have a 5V regulator in this kit I would go for the USB input.