-I set my planes up so normal flying is done with low rates on, When I want extra throw in the control surfaces (movement) I use high rates.

-You are saying you have charged and discharged your batteries, right? What were your findings? Did they discharge to their rated capacity? If you have no means to determine if they have discharged to their rated capacity with a battery cycler, you should get at least 1.5 hours of discharge time, when turned on and you fiddle with the sticks for that amount of time. Most transmitters now show their current voltage in use. When the TX battery gets to 9.6 volts it's time to stop and recharge, when the RX gets to 4.8 volts it's time to stop and recharge. If you don't have a cycler, I would strongly recommend you get one and also an Expanded Scale Voltmeter or ESV. If you can't make any of these determinations, buy new batteries and then get a cycler

-You can use the same fuel. Because most 4 cycles like Nitro in their fuel, 15% is a good blend usable for both.

-Rudder/Aileron mix is generally used to overcome adverse yaw (the down deflected aileron causes more drag then the up deflected aileron when rolling an airplane). Your Extra should not be an airplane which exhibits much adverse yaw. Generally, slow flying high wing airplanes will need this mix, more then faster flying airplanes.