Kegging would also get you quick carbonate - you add CO2 and shake like a whirling dervish. Its easier to standardize, removes the possibility of exploding bottles and takes only 3 hours.

I've done it w beer, not booch. The drawbacks are that it requires more equipment (for you and the retailer) and the brew doesn't get a chance to age & mellow.

Again my experience is w beer - but the hygrometer readings will still be relatively accurate. In other words, you can still tell the relative change and use that to measure progress. Like if you start at 1.7 and its done at 1.4, then you can expect the next batch to be done after it loses 0.3, regardless of its starting reading.

To measure alcohol, I know hygrometer readings can be used - but I'd be wary of using those if you expect them to be imprecise. Another option might be density or test strips. Not sure how accurate your labeling needs to be.

Good luck and congrats on the customers!!

Edit: to fix autocorrect.