I'll start by saying I'm not as impressed with this one as you are. Its a nice airplane, but there are certainly better foamie bipes out there.

no big secrets, you just asked a very nebulous question. Everyone trims and mixes a little different to get the feel they like. some people like to fly without mixes, its all a matter of taste.

Rudder - elevator to mix out the knife edge coupling. I still havent been able to get mine to knife edge cleanly yet or I'd post my ratio... I'm thinking the best is to mix it out of a mid range speed and hand fly it for slow or fast flight as the amount of mixing appears to be different depending on the speed. I have played endlessly with the CG and I've got it close, but too much rudder and bam its hovering. Too much elevator, and bam its hovering.... but it torque rolls nicely Spoilerons and flaperons do some pretty cool things with this one. I reccomend playing with them, but make sure you put the spoilerons on a spring loaded switch, it gets real sqirrelly.

12x6 that came with it is a good brushless prop for 3 cells - like it mostly because its a really tough prop to break, its nowhere near as brittle as the APC or GWS. I'm running it on a razor 400 and a GWS 6.6:1 gear. You could try a smaller diameter prop, but I dont think it would help much. You might try soft start on your ESC and a throttle curve if your tx is capable, other than that, you pretty much have to live with it as the price of being overpowered.