3. Lexical is not strictly tied to collaboration but its plugin system was built to be extensible enough to cater all developers needs. Collaboration is just another plugin (@lexical/yjs) and does listen and perform the conversion every time there's changes in the EditorState.
This model of independent plugins that can be plugged-and-played without further ado also simplifies devX as collaboration can be added later when the application is mature without the need to rethink any of the plugins that were originally created for non-collab plain/rich text.
4. Lexical Web is just the first of many. We want to port the API and fundamentals to various others platforms, including iOS for which we are already performing the initial set of testing. Cross-platform means the API, including Node's will look alike and EditorState and node's properties will be compatible even if behind the scenes the reconciler is constrained by the toolkit available on each platform (and doesn't render or behave 100% like Web's).