I have one of the 35% Giles, as well as the 40%.

The c/g is forward of where it should be. Where you have it works, but makes for a snappier plane. Center of wing tube works a lot better. The forward edge of the wing tube is about as far forward as you should have it. BTW, the cause of any real spin is a stall. A plane will not spin unless it has been stalled first.

All planes yaw with an increase in power. It's called "P" factor and torque. You also have a thing called a "spiraling slipstream" . All of this is caused by the propeller and the engine. If you also had a rudder on the bottom of the plane the issue would go away. If your propeller was to rotate to the right instead of the leaft the yaw would be the other direction from what it is now.

An example of how power levels, torque, and yaw effects a plane was observed this past weekend. A 46% Ultimate at full throttle in a knife edge pass had almost no pitch coupling and flew a nice straight pass. When the throttle was reduced the plane would complete a perfect knife edge circle to the belly without any change in rudder or elevator input. So to answer to your question about if changing power levels could change the amount of yaw, the answer is most definately yes. It also effects other factors, such as pitch coupling.

You will have to experiment with props to some extent to obtain the kind of performance you want. What will make things a lot easier for you, even with the prop you are using now, is to set up some mixes in your transmitter to offset yaw with throttle changes. Use the throttle as the master and the rudder as the slave. Input rudder in the correct direction in very small increments as the throttle is increased or reduced. It will take some time and experimentation but you will be nicely rewarded when you have it right. This is a very common practice in IMAC and other flight competition and is part of the normal flight trimming process.