Is it known in Mythic Europe that ice is less dense than water? ie has greater volume than an equal weight of water?
And that when ice forms, it expands, putting enough pressure to crack pipes and even stone?
Muto magics can expand something until there is no further room to expand.
Does this expansion limit apply to Rego water -> ice?
Anyone with a sufficient score in Philosophiae to know Archimedes' principle (and who's seen ice float on water) knows that.
That water turning into ice can break pottery is probably well known since pottery has existed 
Once one knows that, one can easily conjecture, and probably observe, it might break tougher stuff.
I would point out that this observation does not, per se, rest on an understanding that ice has lower density than water. For example, people know that many plants can, by growing, crack stone. But that does not mean of plants grow by lowering their density.
Unclear. However, I think that if you know ArM5 natural philosophy, you know that turning water to ice involves rearranging its atoms, which is consistent with doing it with Rego. It seems to me that sure, you can use that rearrangement to "push" stuff - e.g. crack a wall by having water seep through its cracks and then freeze it. However, it also seems clear that if you try to give a really big "push" (i.e. crack something really strong) you might need extra magnitudes. In general, I would not allow this to break stone more easily (i.e. at lower magnitudes) than directly using Perdo Terram. Does that make sense?
Granite and other stone had for centuries been quarried with ice- drilling holes into rock faces and filling them with water that would freeze overnight and break away massive slabs of rock. As such I would have to say the principle was well known, though perhaps not widely known if it was considered a guild secret.
The water would of course overnight only freeze in colder areas.
This is the typical problem of engineering in the middle ages: much relevant information is available onty in specific areas to specific people, and there is no widely spread scientific community to communicate it in.
Depending on area it might be a seasonal effcet as well- however ice quarrying in winter at altitude should be possible at most European latitudes, and remember that deserts frequently get below freezing over night despite generally being at lower latitudes because the dry air does not retain heat. There are obviously places this would not work, but these may be places that do not see a lot of quarrying industry as well.
It occurs to me that you should also be able to take this approach with herbam and tree roots (which are notorious for cracking stone).
Is that an absolute? I know some spells have that limitation, however muto being constrained when growing asks some interesting questions.
I muto a huge boulder to a pebble then shove it in a barn smaller than the boulder's original size. At sunset does the barn get turned in to so much firewood by the boulder expanding? If the boulder stops growing when it gets to the barn's edge, we have the peculiar event of a muto spell having a perdo effort, as some rock is now destroyed.
I could make a production line years ahead of time. Have a mould, shrink the metal with a diameter Muto spell, put it in the mould, it expands to the constricting mould, perfectly shaped metal item in 2 minutes.
If muto ending isn't constrained by what it is contained by, why should muto commencing have a limitation? From the Hulk ripping his shirt and shorts open, to Alice cracking open the house, mutations breaking things seems common in stories.
To be fair you could do that anyways- muto the metal into a liquid and pour it into the mold without losing material. Mass production with magic is hardly a challenge.
To be fair, most enchanted items created for production do something that could be used as part of a construction line. In general you are much better served by mutoing something into a liquid if you are going to add it to a mold, rather than shrinking it. Even complex molds which are difficult to do today are easy since the liquid does not have to be heated to melting.