1. each district and overseas command had a set establishment of general officers of different rank, for which the army was willing to pay a salary for. Cockburn's promotion exceeded the establishment of lieutenant generals and so he had to return home. Had the army wished to retain and pay him as a lieutenant general they would have retained him upon the staff in that rank
2. general brevets were announced for different reasons. One was the King's birthday, another would be to move the officers up the promotion ladder and ensure there was a more equal balance between the number in each grade.
3. no, the highest purchase rank was lieutenant colonel.