You have answered your own question there. Jaber Abdullah as far as I am aware does not retain jockeys. Hughes had the ride on Youmzain for instance but was not retained, with, as I am sure you more than anyone knows, Eddie Creighton and Tadgh O'Shea taking the bulk of the rides this term as Channon's (Jaber's main trainer) stable jockeys. In previous seasons the likes of Durcan and Culhane (former stable jocks of Channon's) have enjoyed the bulk of Abdullah's rides, so this indicates that principally Abdullah uses the stable jockey of the trainer, unless he feels a different style maybe required.
In Spencer's brief association with the owner, he showed a 21% SR, a pound level stake profit of 71.50 from only 14 rides. Hardly a disaster. As you said he won a Lowther for him. He also got a vital Group victory out of Nijoom Dubai at Royal Ascot - the filly has never won another race., even with the mighty Ryan Moore riding in the Sandringham. Which makes the decision to replace him on those horses in particular all the more questionable.
Stoute is not adverse for using outside jockeys and I think the fact one of the greatest trainers in racing doesn't use him, displays that maybe the majority don't particularly rate him.
As you full well know Stoute had Fallon retained when Spencer was still at Ballydoyle and Ryan Moore was already waiting the wings as the yards claimer. It is oversimplistic to say Stoute won't use him per se. You know full well he has ridden for the yard in the past. The problem is that with Spencer's own loyalty to other yards, Stoute cannot rely on him to take the outside rides, and like any trainer likes consistency in his riding arrangements. Currently Robert Winston, Richard Hills (Hamdan connection) Jamie Hamblett (claimer) and Louis Beuzelin (claimer) get the bulk of the outside rides. No surprise in any of those then.
Spencer has picked up 3 outside rides this season from Stoute, a similar figure to the likes of Sanders, Callan, Kinane and Hughes. So I think you are going up a blind alley with this, especially when we recall that last term during Ryan Moore's absence Spencer started to pick up a lot of spares from Stoute yielding a 33% SR from 18 rides.