about those timelines, are you serving them from the posts table? guessing you'll be partitioning or sharding that table by blog, right? cool, so you'll need to tree out reblogs which is totally doable in a single query.. hope your sharding doesn't take a performance hit doing cross-partition joins.
or are you going to build a specific service for doing that?
are you using the existing media-management system? so that's a database call or three for every embedded image, video or audio-clip.
or are you going to rebuild that system to pull from a static cache?
how are you storing likes? postmeta? cool, so that's loading a set of serialised strings every post load, that you then have to deserialise to extract a value.
or are you going to store them somewhere else? maybe adopt the eventual consistency model that just about every other social media site uses for interaction tracking?
what's the plan for the editor? tumblr's post editor isn't amazing but it's a hell of a lot easier to use than wordpresses?
see, at this point you're basically building a new platform on top of the, frankly, awful foundations provided by wordpress.
i hate to be that guy, but at this point just rewrite the damn thing if you're that bothered.