Upload & Sell: On
...Do you think the ZE50MP is the best option for reverse mode (i guess the floating element does more harm than good this way)?
I don't know. You can pick up a Takumar of SMC-Takumar 50/4 Macro for about $75. OTOH, you don't have to reverse at 50mm lens to get high mag on a bellows system. Here's a figure that shows what I get with my Mamiya Auto Bellows N. The magnification values would be the about same for any similar focal length lenses, with the same bellows extension (see first figure). The "Lens Mount Distance" in this figure is from the sensor to the front standard of the bellows, where the lens is mounted.
The working distance (from front of lens to subject) really shortens for high magnification, with shorter focal length lenses (see second figure), and so lighting becomes a challenge. You could get better working distance with longer focal length lenses, but would need longer bellows extension, or add extension tubes.
The working distance data shown in the second figure depends on the geometry of the bellows, lens and adapters. The data in the first figure probably varies a bit with lens design (for same focal lengths), but is not dependent on bellows length, etc, as long as it fits between the sensor and subject.