Dedicated Macro Lens On Bellows?

The problem with a lens like your 60mm is that unless you get a very expensive Canon bellows or one of those (also expensive) wired Novoflex reversing adapters, you will lose aperture control. You can do a trick where you set your aperture and remove the lens from the camera with the DOF-preview button held down, but that's really inconvenient, and IMO, crazy.

The alternative is to use extension tubes. There is a Kenko extension tube set which lets you continue to use all the camera controls for aperture and focus and is much cheaper than the Canon tubes. Control of the magnification isn't as straight-forward as with bellows, but you still have the same kind of range because the lens has a focus helicoid.

No idea. Honestly, I wouldn't expect either to outperform a duplication or enlarging lens, which are optimized for a specific, narrow purpose which happens to coincide with the needs of coin photography.

Most older, dedicated macro lenses are designed for up to 1:2 magnification. Those lenses typically achieve higher mags by simply extending the lens, like a bellows would do. To get to 1:1 you need more extension. For lenses (like your 60) that can achieve 1:1 natively, the focal length is reduced at higher mags to minimize the required extension. This reduces the working distance to a large degree and can make higher mags difficult to illuminate, especially since the lens is large in diameter and shadows your lights. For newer lenses, as the 'Cap said, you lose mechanical and electrical connection so AF lenses are difficult to operate on the less-expensive bellows.

You want to reverse a lens when the optical formula favors it, which is usually beyond 1:1. For instance, the manual focus Nikon 55mm Micro-Nikkor works from infinity to 1:2 natively, and up to 1:1 with the M2 or PK-13 extension. At 1:1, You can reverse the lens and get essentially the same image quality, since images on both sides of the lens are the same size. But beyond 1:1, the optics "reverse" their properties, so for instance at 2:1 you can get a much better image quality with the lens reversed than in normal orientation. Your working distance is also improved.

Since dedicated macros (and enlarging/duplicating lenses) are designed to go from flat field object to flat field sensor, you can get better image quality from them than normal prime lenses, which go from curved field object (constant radius or distance to object) to flat field sensor.

My biggest problem with dedicated macros is their size. It's just too hard to get lighting in with large diameter lenses. For higher mags, even small diameter objectives like the Nikon P4/0.1 are big enough to limit lighting!