Personally I think proposal 2 and 3 should happen concurrently. Using the example in the post I would setup a custom feed (that can hopefully consolidate cross posts) for breakfast. I would put pancakes@a.com which subscribes to pancakemasters@b.com I can also add pancakeart@a.com and waffles@a.com. so when someone posts about the best homemade peanut butter syrup recipe that is cross posted to my pancake and waffle communities, I don't get 4 posts about it, I can see it once and choose where to reply (pancakes obviously, I'm a waffle purist).

Community interlinking/subscription fixes a slightly different problem than custom feeds IMO. It's a really good idea, but I would personally still want custom feeds (with the ability to handle crossposts in a customizable way).