I've disabled all the other operators in the GSX Livery Manager and I've also added a Swissport rule that changes its ICAO prefixes to "ABCDEFGHIJKLMNOPQRSTUVWXYZ" so that it ranks highest in the scoring system
This won't work, because by creating such rule ( which should be listed like this "A B C D E F G H I J K L M N O P Q R S T U V W X Y Z" ), you are just giving Swissport a global score of 1 in every airport. Jet aviation is a special operator, so it also has a default score of 1 everywhere, that's why you see a random choice: both have the same score.
To do what you want, you'll have to give an higher score to Swissport, by using rules with 2 letters, like EG for UK, ED for Germany, etc. That would be a bit cumbersome so, my suggestion would be:
- Leave only Swissport available, so it will reduce the installed Simobjects count.
- Explicitely select Swissport in the Operator page of the airport profile.