I think you are not understanding what a fly-away is. It's when it takes off on it's own, disregarding user control from the controller. It doesn't matter what buttons you have set for what modes. If it is not or cannot receive those commands, it's moot. This is exceedingly rare.
People like having manual mode handy for cases of the GPS lock getting squirrely. If it starts to drift or have trouble holding position, you can hit manual and it will stop trying to use bad GPS data. You have manual control with the sticks. If the solo detects the GPS signal is bad (or lost), it will switch to manual automatically. At which point, you again have control with the sticks.
None of this is an excuse for not reading the guides, not watching the training videos, or flying in unsafe or unsuitable areas.