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I’m curious about the control surfaces. Since you have the four quadcopter motors, you could potentially just induce all three of yaw pitch and roll by powering those up.

I wonder if the reduction in weight from the now unneeded servos would pay for the extra battery drain.

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Great question, but spinning lifting motors in cruise draws more power than it's worth for efficiency.

There's ways to use differential thrust for multiple cruise motors to roll if not positioned at spanwise Cg (e.g. wing mounted like in a commercial jet), but again, usually not efficient. Servos are a small mass fraction relative to other components.

How would you get yaw?

Same way you get it in a quadcopter. You increase rpm in the (say) clockwise rotating props and decrease it in the counter clockwise rotors.

Net effect is yaw counter clockwise, but zero net change in lift.

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