Would love feedback on the approach, method name(s), and whether or not I should be able to get the left / right side diff IDs from $request before I deploy this a hojillion places.

For consistency, maybe call it willRenderTimeline()?

whether or not I should be able to get the left / right side diff IDs from $request before I deploy this a hojillion places

Oh, yuck. You probably don't have enough information to figure it out unless you add more information to the AJAX request.

Maybe the method needs to let you pass extra data, which is embedded in the timeline on the first render, and then passed to the server in the AJAX request so the callback sees the same view of the world in all cases.