That 1/2 gal addition should give you an adjusted OG of between 1.115 and 1.12. I do 2 to 2.5 gal batches - the 2 gals give me enough to drink and share while affordable enough to make a variety of different kinds. I usually split the batch to make slightly different versions, commonly oaking just one gallon. The 1/2 gal is to mitigate racking loss.
When ever I use 71B or d-47 (which is most of the time) I begin with an OG of 1.12. It routinely ferments dry. By adding a 1/2 gal of water to your must, I think your current gravity is down to at least 1.050. If you can coax 3 more clicks you'll be at a very tasty 1.020.
Continue to stir to keep the yeast off the bottom and maybe they'll be encouraged to produce more.
If this doesn't work out, and your yeast craps out and you're left with a too sweet mead, you could make a gal of real dry traditional mead - just honey and water - with d-47 and mix them. That would work.