More questions from the newbie

Q4: I plan on setting the throttle and servo system to where when I put the throttle trim all the way down and pull the throttle all the way back, it closes the throttle completely and shuts off the engine. The problem came when I found out that when I put the throttle trim all the way up (as to not shut the engine off when I pull all the way back on the throttle), and pushed the throttle stick all the way up, the servo wants to move the throttle arm on the engine past it's upper extreme. Basically, there is not enough slack. Is there some device that can solve this. I was thinking I could implement some type of spring system to where when I push the throttle stick all the way up, the spring will take up the extra movement and not drain the battery.

Q5: The engine instructions said not to use anything over 15% nitro and less than 18% oil until the engine is broken in. I used exactly 15% nitro and 18% oil when I broke it in today. Is there a problem with this. I used it because it was what I bought before my hobby shop guy told me that he usually uses 10% nitro.

Q6: The engine instructions said that the midline for the fuel tank should be 1/8"-1/4" below the fuel line connection to the engine. It is difficult for me to do this because the engine is upside down and the fuel line connection to the engine is quite low. The fuel tank would need to be all the way to the floor of the plane (if even possible) for me to conform to the instruction. Would it be a problem if the fuel tank midline is level with the connection or even a little bit above it. What is the reasoning behind what the instruction says?