First of all Itzhak Perlman and Hilary Hahn are masters who have developped their playing skills far beyond the basics, but they still know their basics and there is no doubt that their technique has a solid base.
I think Itzhak Perlman's bow is straight. When it is not straight it is build upon a straight bow as the base as far as I can see.
Hilary Hahn's wrist is certainly flexible, it can sometimes be with rather small movements in the wrist, but the flexibility is certainly there.
Regarding Eddy Chen's video: Note what he says at 3:50:
Everything has a context when it comes to the bow arm
That sentence is interesting.
In relation to that I will say that the more you learn about bowing tecnique the more you learn about an incredible amount of possibilities that exists. And that enables you to play with a great variation in dynamics and expression. But in the beginning a main focus is to play with a good tone. Then you can expand your possibilities from there.
Regarding playing with a straight bow:
Well, you can not play with a straight bow without some wrist flexibility. Hilary Hahn's wrist is indeed flexible as I wrote above. Her finger flexibility is sometimes subtle, but it is there. From 9:30 to 9:45 her finger flexibility is more apparent.
You asked this question:
What is more fundamental to tone production--straight bow motion, or flexible fingers?
I teach violin. Beginners often have a tendency to hold a firm stiff grip on the bow at the start. You do need to loosen it up, but finger movements is not the first thing you introduce to the beginner. Instead you teach them to hold the bow somewhat softer and to be a little flexible in the wrist which is needed in order to be able to play with a straight bow. In this process the fingers won't stay completely stiff anymore, but the type of finger movements that Eddy Chen demonstrated in the start of his video is introduced much later. His focus on the arm is good, although it can be a bit much for a beginner which means you will often start with smaller bow strokes and include longer bow strokes gradually.