"Doesn't it feel like Fischer would have an edge in the trial where they both had equal old school training?"
I think such things are more or less impossible to speculate about. Fischer and Spassky both had old school training and after seven games between them Spassky had +5-0=2. After the 1972 match finished Fischer was up 7-6 in wins.
Carlsen has often talked about how little influence computers had on his development, he for a long time mainly studied the old school way and his early coaches were amazed at his lack of computer work.
Would Carlsen beat Spassky, all things equal? Also difficult to say, but when experts rank the greatest World Champions the latter often ends up in the bottom two with Euwe. As World Champion he was never ranked #1. Carlsen on the other hand has been clear #1 for more than a decade, and he faces opposition that is far from old school. Maybe he would do better against Fischer than Spassky did.
I've always felt that Spassky was horribly underrated in terms of not only his Strength as a champion but also his achievements as a theoretician.
Now Botvinnik on the other hand...
its war worse than that lol, fischer literally said that spassky is the strongest player he ever played against. tal also said that during 1970 or something, spassky was 100% the strongest player alive. spassky aint in the top 5 strongest world champs imo, but he should be in the top 10, instead of the bottom. the guy was complete monster.
Carlsen has often talked about how little influence computers had on his development
I didn't know this (again I am a clueless newb). That's a point for Carlsen. Still, he's been learning a ton all of his life from matches played in the modern era. Regardless whether he studied much modern theory / computer stuff directly, he certainly sponged up benefits by being exposed to it.