I think you are confusing the fact that damaged kidneys spill protein on a routine basis, with your kidneys spilling protein when put under extreme stress.
It’s normal that under stress - even exercise - the kidneys spill more protein, even though it is usually lower amounts.
If your dry fast results in (more) protein being spilled on a average, normal day, then it is doing damage. Otherwise there is no evidence that it has done any damage.
I think you are doing the right thing though in drinking when you’re body requires it.
I don’t know. I wonder what people follow as I don’t find much information on dry fasting on the Internet. I joined because I am a Muslim and I dry fast every Ramadan, but the longest I had to dry fast was 17-17.5 hours when Ramadan came in summer, and then we eat and drink freely entire night before fasting again. What people do here concerns me very much, but they often get upset if you tell them they are dehydrated or in danger. In Islam it is forbidden to dry fast more than two consecutive days without at least drinking some water!