In answer to your first question on topping up, you have a potentially fatal issue in that oxidation will begin when air (oxygen) comes in contact with the wine. It's not urgent in that the wine is protected as long as fermentation continues and CO2 is being produced. In fact you're best served by leaving it alone. (Adding a gallon of wine will increase the alcohol and possibly stall fermentation.) Don't open the carboy and when fermentation ends top-up with similar wine as part of your process. When you top-up that much volume, do it when you siphon rather than pouring it into the carboy, which will introduce air.
On the second issue (wine shortage) is your carboy 5 gallons or 6? Also, it's a silly question, but how do you know you made 5 gallons? (For example, some wine makers mark their primary fermenter at the 5 gallon line.)