This seems to solve the problem of allowing the “old” URLs to work, but it doesn’t seem to solve the problem in which RSS for Entries (Posts) acutally points to RSS for Comments.
As flavien mentions above, setting Permalinks to default allows the main RSS for Entries (Posts) to point to Posts, but even with the fix added to $comments_rewrite (ticket 2379), permalink settings other than default “breaks” the Main RSS Feed. By “breaks”, I mean that the Main RSS points to comments rather than posts.
I’ve implemented the code fix to /wp-include/classes.php, but it doesn’t seem to fix the RSS issue which is the original topic of this discussion thread.