Sure. While it offers little-to-no strength increase, the added weight isn't much to worry about either. Nutted axles offers a tiny theft deterrent, both in terms of removal and in making the wheel look cheaper.

So why not leave it there? Axles are inexpensive enough. Hardly worth scavenging if it means taking apart an otherwise functioning wheel.

No way to tell, too many unknowns and too many variations on diameters and threads.

Not having a pic of the actual item makes the guesswork even harder. But I've never seen an OS axle that would take a regular sized axle inside. You might be seeing some spacers though.