So here is my answer, please continue reading after my initial statement (or don't):
This is all a really bad idea. Being able to freeze air and make sculptures is one thing, but the implications are another one. If one could really just alter properties of individual atoms/molecules just like that, making ice cubes is the last thing anyone would ever do with it. The things you could do with this are endless - from powering near-lightspeed spaceships to killing millions of people with the blink of an eye. I do not want to go into the details, I already discussed some problems in the comments - but basically if you introduce those kinds of magic into our real world, things get messy. Luckily, there is still a chance for you:
You said that your civilization is rather primitive. So if you let's say alter how things work microscopically, they will not notice. Choose your battles. If you are doing iron age fantasy, you do not worry about general relativity and what the result of the Stern-Gerlach experiment would be. Your people live in a strictly Newtonian world - even further, they do not live in a world made out of molecules and "atoms" but in a world made out of water, ice and rock. This is all you need to stay consistent with. If you want "real physics", well, just do it. You already said that you know how to implement it and if your people rather build ice structures than kill each other - well, good for them. But be aware that things do not make as much sense anymore (paradoxically).
I think it is clear that "ice" needs to be what some people might call an element of your world. A state anything can enter and leave if given the right stimuli rather than a marcoscopic observation that is a result of microscopic behaviour.
You should still worry about the pressure changes resulting in those escapades since even primitive cultures for example knew how sails work and certainly have experienced wind, but you could alter how density works and that things become heavier/denser once they enter the "ice state".
Some things are the result of what we call quantum mechanics. Quantum mechanics is completely lost once you say that you treat individual atoms and alter their speed. So do not worry about being consistent with our world. There will always be magic to save you - sure water ice does float on liquid water - so maybe do not change density in general but introduce magic density for example or remember that water even in our world is more of an exception than the rule - or as a third option make the liquid and ice state denser. There are many options once you backed yourself out of that corner you are in currently.
And the details - how do proteins work in a Newtonian world? Well, your culture still doesn't know what proteins are. There are many details like this and many things to clarify, but I would heavily advice you against trying to just introduce ice magic into our world and expecting that things still make any sense. Often enough, being super vague is a solution btw. Even pretty hard sci-fi technobabbles over all the important details. At the end of the day, all humans live in a Newtonian world made out of rocks and air and not in a quantum mechanical nightmare - that is how we see the world. This is how your audience knows the world. Introducing weird concepts to be consistent with contemporary physics is helping the 0.01% of humans that understand that stuff (I'm not one of them myself) - and even they think mostly in ice and rock. Choose your battles wisely.