My vet always told us to keep lots of water handy and use dry food because it keeps their teeth cleaned better and healthier than wet food. It's so darn confusing sometimes. It seems different vets have such differing ideas.
I give my cats soft food as a treat but they always use dry Purina One Hairball formula. They have done well on it. They drink a lot though and so I have to fill the bowls regularly. So, that's a good thing!
Until recently it was believed that Ash caused stones. crystals in cats. recent studies have shown this to not likely to be true.
As far as salt goes, that really depends on the ph balance of your cat. So while salt itself doesn't cause stones, depending on your cat's urine ph balance, it could be a factor.
And yes, the changes you mention can cause stones, but it's generally more because of stress related to changes.
As to diet... cats get 75% of their hydration from their food. So a good, healthy wet food is crucial to diet. One of the biggest causes of crystals is a dry food only diet.
Your best bet is to go with a good, high quality wet food and as little dry as possible.