Certainly, the AVR single ADC converter uses a sample and hold capacitor that is switched to the pin to be measured by the input pin multiplexer. The 'charge time' for the S/H cap is effected by the source impedance of the voltage trying to charge the cap.
Sure, but for slowly changing signals the charge on the capacitor will be about the same between each sample. You don't need much source current unless you're trying to change that charge by large amounts on each sample.
This is all just pontificating of course, but it's easy/cheap to try it and see what happens.