@vij You might want to reach out to email@example.com
I don't know their plugin pricing but those features don't sound like too much effort. And if you allow it to be licensed at their will (please be more precise on this, "sell them to others" might be more restrictive than you want to be) it might give you bonus points
I myself don't have much spare time right now, so I'm out of this.
As it says he still has 450 posts, but only posts/topics as his current username appear and searching doesn't seem to find much
can you PM us a link to his user profile? sounds like a bug, but as @baris says, all posts are tied to UID so changing username won't make a diff, but possible something else (deleting posts?) could affect that. Tried fooling around a bit on my dev env, couldn't repro so I don't really know.