weight is your enemy...I have only had 1 servo fail on me in flight, and it was a standard plastic gear servo, gears stripped....get a metal gear servo that packs enough torque for your gasser...and only use 1 since your elevators aren't split..save the weight.....if a servo is going to fail it could just as easy be your rudder servo causing a hard over rudder...you won't save it from that.....If it was me I'd Y the ailerons and save the channels...
Good luck