So how does one get into scion, when there are no active imms and no leaders? Do we still pray?

Play like you would, regardless of the eyes that may or may not be upon you. If it is suitable for your character to pray, then do so, and if not, then don't. The same goes for the note to the cabal. I'll admit that from past experience, this isn't an easy path and in generally has taken me considerably more time to accomplish when I've done so but the random interaction and induction because you're doing the best that you can do, as your character, really does pay off in the end. If Scion is your goal, good luck, and stick to it.

True, but who does the inducting? Random imm from any cabal?

To be Honest, I'm not entirely sure how all that works but I do know that some of my characters have had observations posted by IMMs that were not in the cabal that I was playing that recommended advancement, induction or reward for game play. That being said, I'm sure that they communicate such things to one another.