Q1, Use the higher amperage supply.
Q2, far more complicated
Broadly speaking, voltage controls max speed, where current controls torque. The torque load on a fan blowing air is going to be less than that of a liquid mixer for a given speed (viscosity differences). This is all to say you are going to be using the fan well out of its design spec, this is not inherently bad, its likely just going to be inefficient and not necessarily work the ways you would expect.
By using a potentiometer you are more directly controlling voltage than current, it's not going to be as easy to predict what potentiometer will work best. I will note you will need to use a relatively high wattage resistor, the fan you spec is a ~10 Watt model. To get low speeds you will have to sink almost all of that power into your resistor. 10 Watt potentiometer are going to be expensive. A better, more controllable option is to get a variable voltage supply, or en even better option is a variable current supply (probably not possible.