Reading your post made me think it would be nice to have a touch-button on every tiddler - then touching a tiddler that has not been touched before creates the two new tiddler fields with sensible defaults which can be fine tuned at leisure ( say an initial review-interval of 30 days ) - tiddlers that have the two fields are considered part of the “pool” and ones that don’t are not.
Remove Tiddler From Scheme?: I would probably opt initially for the rather unsightly manual option of edit tiddler and deletion of the two fields to remove a tiddler from the review scheme - yes untidy and not polished but I would want to avoid removing a tiddler from the pool accidentally whilst scrolling up and down the story river. ( by means of example I do occasionally accidentally remove tiddlers from my favourites list - one click solutions are convenient but also convenient for mistakes - perennial usability problem ) - otherwise I end up writing “Are you sure?” conditionals - so my crude method would be unsightly YES inconvenient NO.
I guess one reason I am not so attracted to the more sophisticated and engineered solutions available is that my tiddlers vary from single paragraphs to articles, essays and whole chapters from text books - so not really a binary Q and A situation or exam preparation situation. I like simple solutions where possible and so the idea of being able to populate the story river with the ‘N’ tiddlers currently most requiring a re-read appeals to me most.