Choosing to use water in your must is completely optional. Some fruits, like blackberry or black currant, are cut with water to help balance the acidity; though many do make these as no water added meads too & adjust the acidity in otherways. 100% juice wines or combo of 100% juice + fruit are also made. It all boils down to what works for you.
In regard to the ascorbic/fumaric/citric acids you may see on labels, those doses are typically never an issue when it comes down to fermenting with a wine yeast.
I wanted to make sure I read your post correctly. Are you saying you used 5 gallons of water and then added 18# berries with another 5# in secondary? What was your initial yield from the 18# of fruit in primary?
Motto: quel che sara sara