I am not talking about nibs that are noticeably scratchy, but nibs that aren't as smooth as they could be. Can they become eventually a little smoother through extensive use?

Quite likely - after 50 years they will be as smooth as the Conway Stewarts and Burnhams I have.

Otherwise, I would seek out some pieces of Micromesh cushioned abrasive material - it comes as fine as 12000 grit. Just draw a few 8s and clockwise/anticlockwise circles on a small sheet and you'd be surprised at the immediate improvement.

Also a brown paper bag, or the paper that you get in banana boxes may also work, although you will need more 8's.

After very long use, yes. It should take years to improve.

Yep, but like noted, it helps to burnish or otherwise help the process...

Bill

Be very, very careful if sanding a nib, even with a paper bag. I'd check with one of the local experts before doing so. Asking their advice saved me from ruining my pens that way.

Yes!