Don't do that, what if the wine takes on the taste of the baloon?
Rubber and wine aren't normally in contact with one another for any
length of time. I think a safer bet (if necessary) is to use a similar
wine, you would not be adding much at all.

Don't rack until there is a reason, I'm not sure how you ended up
losing 1.5 gallon in racking unless you you were pressing off the
skins. 1/2 gallon to top should work out well for you; it's 10 %.
Make sure that half gallon is topped up too if you want to stick with
only one variety in this batch.

Patience is a virtue, especially in winemaking. ) You should start
to see the wine clear in the neck and striate soon if it's done
fermenting; maybe a month from now it will look mostly clear, it will
look darker where it's clean. You can use a flashlight to see how
things are going. Look for the beam itself; if there are suspended
solids you will see the beam reflect off of them.

Joe