Just to touch on some of your points:
Pre-production units being used for reviews is nothing unusual. OTOH, the Xtrfy M4, Glorious Model D, and HK Gaming Mira-S/M all were pre-production units, yet all of them got an Editor's Choice. On the other hand, I've had retail samples with much greater issues than the Zephyr, issues grave enough that I had to request a new sample. Whether a sample is suitable for review can only be determined on a case-by-case basis.
Not clicks, just steps. I.e., when scrolling up (not down) by a single notch, in one out of 20 cases said notch wasn't registered. It's certainly quite irritating, but considering the specific conditions that need to be met for this to occur, I'd say it's forgiveable. Speaking of other brands, the scroll wheel on the Logitech G403 I'm using for my comparisons either doesn't register steps at all or reverses the input (i.e., when scrolling up it scrolls down on screen). As for the mouse being 'mediocre', sensor performance, mouse feet, button quality, and the cable certainly elevate the Zephyr above mediocrity. I only know a single mouse with a 3389 for $35, which is the Dream Machines DM2 Supreme. Most mice with this sensor cost $50 and above.
That has me a little concerned as well, as mentioned in the conclusion. For what it's worth, I'll be getting a finalised sample in October (target date), so I'll be able to verify whether the promised improvements indeed have taken place. If the issues persist, I'm certainly open to revoking the Recommendation.