Quickie clarifications for those following along...
Yes, per @jefito, Conduit is common *plumbing* on the client-side.
Conduit has no relation to the UX. Every Evernote client built on Conduit could have a different UX and Conduit wouldn't mind. Users might mind, and we would definitely mind, but Conduit wouldn't. That's basically the whole point.
Where Conduit figures in is anywhere that information flows between client and cloud - whether that information is content moving back and forth or a service call of some kind. We expect all of that information to flow through Conduit (on the client side). The goal of conduit is to have common functionality behave in consistent and predictable ways across all of our clients. Sync is the easiest function to understand this with: we'd like all clients to sync a recently modified note (whether that's sync up or sync down) in a consistent way (allowing of course for variances in whether or not that particular note is supposed to be synced to that device).
To the concern that @CalS raises about local data stores, part of Conduit's job is to interface to the appropriate technology that forms the local data store (which, unsurprisingly, is a pretty different technology layer on a browser, mobile device or on a PC). That, along with the different memory / CPU / power profiles of the different devices on which our clients run, drives the need for per-device optimization that we talk about in the video, that makes Conduit such a beast to get right.
And finally, Conduit, in and of itself, does not close the door to device-specific customizations or integrations. To the extent that device-specific functionality needs to get to the cloud, it will have to go through Conduit at some point. But the functionality that fits that description is usually travelling a well-trodden (ie. pre-existing) path on its way to the cloud, rather than having to carve its own unique API pathway.
As @CalS says, yes, there are so many questions. But I do not have time to answer them all (nor, quite honestly, have we actually converged on some of the answers yet - if only because hard experience teaches one not to constrain oneself to an answer before you actually need it). Hopefully the elaborations above provide some clarity.
Back to lurking,