How does it do that?
The duration of tenure prevents accumulation of power in an individual, Power is spread and the period of tenure should be short enough to weed out those whose chief ambition is power.
How does it reduce the opportunities for patronage?
It reduced the power and extent of the patron, again weeding out the ambitious.
What if the moderator is stretched beyond his or her capacity
That is a universal problem for anyone in a role but a moderator would not have as much scope to mess up as an A oC
Why would changing the name of the role of CofE "chief executive" help in any way?
It wouldn't just be a name change( see Role of the moderator of the Church of Scotland). I think CEOCOE is a bit of a crude analogy but I can certainly see how Welby's tenure might have given that impression