The only time you need to minimize headspace is if you are moving the beer to a secondary vessel (for bulk aging or other purposes) because there likely won't be enough fermentation activity to consume the O2 introduced during the transfer. However if you flush the receiving vessel with CO2 and do a closed loop transfer then I don't think head space would be an issue there either.
The pH of the water is not that important, as it has little buffering power: just a small amount of acid or base will change the pH easily and significantly.
Absolutely, pH of beginning water is relatively inconsequential, I personally use acidulated malt to get the pH down into range in light colored beers. Usually only 20 to 80 grams, or less than 1% of the grist. In a dark beer, you typically need the opposite, baking soda will bring the pH up into range. I used under 4.8 grams in the stout I brewed recently
We all drop one or two periodically. I did a 70 shilling ale a couple months back and everything went smoothly, so I though. I do a late addition of some dark malts- we're talking ... 4(?) ounces or so. I thought I did a late enough addition but I thought wrong. I put it back on the schedule a couple beers later and paid diligent attention to keep the dark malts in there no more than 20m. Score. Just the right amount of 'character'.
I can see this theory playing out. Where would oxidation most likely begin? I ferment in a bucket with airlock. I did remove the top at 10 days to take a gravity reading, making certain everything was sanitized, and then re-topped with the airlock until day 14 when bottling took place. For bottling, I siphoned from the fermenting bucket into a bottling bucket with the pre-measured corn sugar solution. Bottling and capping was complete within an hour.
