DY: The Darchei HaLimud are based on Mesorahs that have a Makor in Shas and Rishonim.(‘???)
Precisely true, but you don’t have a Gemara which explicitly says it; you only know that a particular mehalech is valid because you’ve been mekabel from your rebbeim! Shmuly Yanklowitz can also claim a derech halimud which goes back through the Rishonim to the Amoraim; he wouldn’t be wrong because mesorah doesn’t work that way, he’d be wrong because he made it up. Can we really trace the Brisker mehalech to earlier than R’ Chayim? No, but we trust R’ Chayim (why, is a shmooze for itself) that despite it’s seeming chiddush, it’s solidly based on mesorah (ang then maybe we can even go back and find remazim).
You want to know why I disagree with you so stubbornly? Because I think the approach which ignores the unwritten mesorah, especially about “intangibles”, is precicely what created the YCT monster. A lot of their credibility (to some of the hamon am) comes from the fact that they studiously avoided (until recent developments) saying anything directly contradicted by the sources. Those with a strong, unwritten, mesorah, discredited them immediately, but some of their poison has seeped in due to the fact it’s taken too long for some, who don’t have or appreciate a strong, unwritten mesorah, to reject them.