With the current widgets I can make a user timeline on the fly when a page is loaded for a particular user. With embedded timelines this does not seem possible, because they need to be pre-configured. Is that correct? Is there a way to show an embedded timeline for a user when a page is loaded asking for a specific user?
There is no means to generate timelines on the fly at this time. To provide a service like that, you’ll need to implement server-side means to accomplish this with the REST API and your own design skills.