] My sense is that the bishop's right of installation was for practical purposes never exercised contrary to the patron's will, except perhaps in the case of a notoriously scandalous candidate.
] "Never" is a strong word. What evidence do you have that Bishops never excercised their right contrary to their patron's will, except in one case?
In 30+ years of off and on reading of 18th century English history and literature, I have never come across a reference to this happening. On the contrary, the unquestioned, indeed unconscious assumption is that any ordained man the patron wishes to present will be installed. The 18th century bench of bishops, moreover, were very much men of their class, appointed for patronage reasons, and well integrated into it culturally and socially. While there are no doubt individual exceptions, they were as a group not noted in retrospect either for zeal or for sanctity. All appointive office in 18th century England tended to be regarded as property. If you have instances that ecclesiastical office was treated otherwise, I'd be interested to see it.
] The quality of the Anglican clergy in the 18th century was too low, and the English respect for both property and patronage too high for it to have been otherwise.
] On what basis are you making this assumption? Are you sure you are not allowing accounts of the bad few to drown out the mostly silent voices of the many good? How are you evaluating the "quality" of the clergy?
Again, on the basis of many years of reading. The eventual rise of the Evangelical movement was very much a reaction to the pluralism, non-residence, pursuit of non-religious interests and overall Laodicean quality of the 18th century Anglican clergy as a whole. So was the Methodist movement. The success of Methodism in particular tells me that there was a broad spiritual hunger that the Establishment was not satisfying. The overt hostility of many Anglican clergy to Methodism tells me that they were far more concerned to protect themselves from competition than they were to satisfy this demand.
From the viewpoint of Wilberforce or Wesley (which I do not share), there was something quite wrong with a culture where no one thought it improper or unusual for a modest, retiring young man of good family like Edward Ferrars to take orders simply because the Church was a quiet, respectable alternative to the Bar, the Army or the Navy. That the Evangelicals had to fight hard for a generation, against widespread opposition, to infuse their views throughout the Church tells me that the conventional picture of the 18th century Anglican Church as a compound of the official, the tolerant and the torpid has a lot of truth to it.