Mention an outside payment system for subscription in an iOS app
What you suggest will get rejected very quickly. I read a quote from the guidelines somewhere (sorry, can't pin it down) that included a phrase about not being allowed to induce users to use another, external, payment system of any sort, which would apply to your various mentions. This is exactly what Spotify ran into recently, they removed the ability to subscribe from inside the app, and put in a page mentioning that there was a way to subscribe outside (with no direct "go here, do this"). They also (separately?), asked for an email address (to which they could eventually send subscription offers, though they made no mention of that). Apple shot it down, Spotify complained very publicly, Apple's lawyer responded publicly.