sorry jimrod; been away a while.
I was asking about pH initially because your mead stopped so early. You should be monitoring it during fermentation to ensure your yeast are healthy. It is generally not a problem with melomels but I wanted to make sure it wasn't simply too low for the yeast to keep working. In such a case pitching more yeast wasn't going to help.
As it turns out, your pH was plenty high and from the immediate re-start upon dilution it seems pretty clear the alcohol tolerance of the yeast was simply not as high as we assumed. I have no idea how low of pH this yeast can handle, fwiw.
as for "optimum" pH, you seemed to be talking about for drinking. I was talking about fermentation where I aim for the high 3's. I get the yeast happy and worry about adjusting later if necessary when it comes to drinking. And stability aside, the exact number isn't as important as whether the acidity is in balance with the other aspects.
for bottling, below 4.5 will inhibit some bacterial growth, especially below 3.5. above 10% ABV will also do a lot. Below 4, 12%, kept cool, sulfited; together I'd feel good about bottling. With significant residual sugar and some nitrogen still in there I'd need a lower number to be comfortable than, say, a dry show mead.