Yah. 2 is the best option from end-user perspective: I really hate it that we eat up subscriptions with notifications that used to be 'free' before ESN, and anything we can do to keep from doing that is a good thing.

I think, though, that doing 3 (or 2 leading to 3, like Fu says) is a good way towards improving the ESN system really well for future work. There's a bug in somewhere to upgrade to LJ's latest version of ESN (since they redid a bunch of stuff and made it possible to email only comments, not inbox required) that you might also want to look at as a starting place, since I don't know if it would be useful at all.