Whilst I understand what you mean ( I hope) there are some possibilities for misunderstanding in your post. My remarks in italics and take them with a grain of salt, I might be wrong ;-) Buen Camino, SY
In 1957 there was no requirement to walk 100 km for a Compostela. - Correct!
There may not even have been a Compostela, but if there was, it was given to anyone who visited the tomb of the Apostle. - Correct, the 100/200km requirement was introduced for the Holy Year 1993, before that indeed everybody that showed up in the Sacristy of Santiago Cathedral got a Compostela (which was far more simple in design) no matter how they got to Santiago. As the old saying goes, it was the thought/devotion not the matter of transport that counted.
The distance requirement is a modern modification according to the Cathedral website. The Camino barely existed, and the number of pilgrims was in the dozens, not the hundreds of thousands. - Correct, apart of one point, the Camino never went into non-existence (that would have been difficult!), but it was, nearly, forgotten over centuries, but it was and is still there, waiting for those that want to take this road ...
Without the advent of the 100 km requirement and the Compostela itself, it is hard to imagine that the Camino would be what it is today. - That is were I strongly disagree, many, many of us have and will walk the Camino without a 'Compostela reward' at the end. We collect it when it is offered, but we still would make the pilgrimage if it wouldn't be offered. The Camino is so much more than a piece of beautifully designed paper ...
With no prize at the end there would not be the Sarria Surge. - Correct, without a Compostela things and pilgrims would be more widely spread.
Without the prize at the end, would the 8% of pilgrims who walk for cultural reasons be almost the only pilgrims? - That is where the 'religious/spiritual intention' has become a self-fulfilling prophesy. To 'qualify' for a Compostela you have to tick the spiritual/religious box on the piece of paper. As the Compostela is offered, 'you' tick it if you want it. And the Pilgrims Office can show in their statistics that nearly all pilgrims did their pilgrimages for spiritual and/or religious reasons. A true kettle of worms if we think more closely about it ...
Would MacLaine, Kerkeling, and Sheen have had anything to do with it? - I think yes, like all pilgrims they had their own reasons to embark on this pilgrimage they way they did it. MacLaine did the way before it really became world famous, Hape Kerkeling published his book many years after having walked the Camino (publishing his book might be influenced by the fame of the Camino in Germany btw but I do believe that when he set out he did it for his personal reasons), Sheen - don't know too much about his/his sons motivations, but it really does seem that the idea for the movie and the idea of pilgrimage occurred very closely together in time.
The questions are rhetorical, by the way; there is rarely actual evidence for speculation on what might have been! - I just added a bit more evidence and a whole lot more of speculation ;-)
Buen Camino, SY