“My understanding from reading this is that because it's an amendment to the developer agreement, that is probably per developer. So you can't opt in some of your apps, opt out some of the others. And right now the documents Apple released say it's a one-way decision.”

The new terms are likely per-developer (not per-app) and probably irreversible

Two huge structural details developers miss: the choice binds your entire account (every app you ship), and Apple's docs suggest it can't be reversed. That makes the decision tree dramatically more consequential than headline analyses suggest. Treat this as a one-way door at the developer-account level, not a per-app experiment.