Tom,
I'd vote for Mr. Bell since he was largely responsible for taking Thomas's courses to another level, while also doing some outstanding work on his own. The combination of the two men and their crew was something special as evidenced by their five most revered early works, La Cumbre, Ojai, Bel Air, Riviera and LACC North (the redo). Brilliant strategy, amazing construction work, all so sound and at the time (or since!), unlike anything ever built before. Bell's work on his own appears to be just as attractive and artfully-constructed, though lacking the strategic flair that the Thomas courses demonstrated.
Ian,
The club's own archives include many of the articles I dug up and which they had for years. They spell out this scenario which is repeated in the club's own history by Robert Windeler: the current site was originally Fowler routing/design with Thomas carrying out the plans on site around 1921 when he was the new member in town, with Thomas-Bell renovating the courses significantly starting after Riviera was complete and ending with Thomas's passing before he could redo the South Course.
To say that Thomas was carrying out Fowler's plans in the late 20s is wrong. It would mean that Thomas's article about the "multiple courses within the course" is pure fiction and that he had many collaborators in the local media mischaracterizing his efforts. I know from watching the "restoration" at LACC that this information wasn't of much interest at the time of the project. It's too bad, what a wasted opportunity to restore something unique in the world of golf.
Geoff