Salinity of brine for feta storage
Feta is very salty. Recipes I've used called for a brine of around 12%. 

Water is fine. When making feta from scratch it helps if the brine is a little bit acidic so the surface of the cheese doesn't soften. It might not be an issue when replacing brine for finished cheese.

https://fankhauserblog.wordpress.com/2007/07/14/feta-cheese/

I've had my own still be good after a month in the fridge before it was all eaten. I would be surprised if it wouldn't keep for 6 months or more. It is a soft cheese and when it is removed from the brine it only gets about a week in the fridge.

This answer: https://cooking.stackexchange.com/a/24152/2001
Suggests that you can put feta in a brine or milk for up to 3 months.

Yours was partially exposed to the air and then will be re-submerged. My suspicion is that the brine will be strong enough to prevent spoilage but there are so many variables in your case that I doubt we'll find a definitive answer.