Super clean and perfect husbandry. 90% of all sickness in reptiles can be traced to husbandry causes. There are exceptions but if you count quarantine as husbandry it reduces again.

Most bacteria do not do well in dry environments most fungus the same can be said virus don't care at all. High humidity and poor ventilation stagnate air are know to increase the bacterial fungi that can cause RI enough ventilation is important (and the thorny part how much is too little??) I use the one change an hour rule (very very low by human standards) The test I use is dry substrate and no animal measure the RH and then remove the water bowl the RH in one hour should be close to the rooms RH. If it is still quite high and the enclosure has no source of humidity I'd add more ventilation. A poor shed here and there is better than a case of RI.

RI cases need to be cultured by a vet and treated with antibiotics that have been proven by the culture to kill that particular strain. With the number of bacteria (5 followed by 30 zeros) a culture is very important guessing and using a broad spectrum may not effective and sadly after the antibiotics have been given the results of any culture are tainted. Cultures need to be done before any drugs are given.

I would not mist for ri I would allow the enclosure to dry out some. not lots just some.

I have no idea about soaking but stress is never good and I would be concerned that adding stress to be worse than any benefit may or may not be gained by soaking. Some snakes tolerate water better than others it depends on the snake. I would likely avoid it myself. Especially if we speak of balls. Other snakes may need to soak I can only really speak about royals.