What are the effects of heavy cream and milk in ice making?

The role of cream in ice cream is, well, to be the main ingredient. This glances over a lot of details, but that's the gist of it, to the point where frozen desserts made without cream are not considered "ice cream".

Getting the proper texture of ice cream is not easy, and it only goes well when you start out with the right conditions. Part of it is to have a proper ratio of liquid to total solids, and of fat solids to nonfat solids. Using cream only will give you too much fat, so milk's role is to adjust the fat content so it falls into the optimal range.

There is no such thing as "more milk makes it smoother" or "more cream makes it smoother". Each good recipe is engineered to have the optimal ratio of ingredients, and if you start changing it on your own, the results are likely to get worse, not better.

Freezer temperature certainly affects consistency, but I can't tell you the interaction between milk/cream content and temperature. In practical terms, if you don't have a dedicated freezer you can set to an optimal temperature (about -6 C for most ice cream types), the question is moot anyway.

Fat is one of the key ingredients that makes ice cream smooth, heavy cream is 35% fat or more, whole milk is 4% fat, therefore adding cream cream makes ice cream smoother than milk. That recipe is about 25% fat from the milk and cream, which is about as low as you want to go, reducing the cream and adding milk is likely to make it harder.