First regarding freeze transformations: They are also good in the following (not to uncommon) occurrance.

Let's say you want to make a chain of elongated spheres (for instance arms). You scale all the arms in one direction (let's say the Y-direction) and then parent them to each other.

Evereything looks fine until you rotate one of the children. Then it becomes shorter and fatter.

This is because the scale-transformation is inherited from parent to child, with the childs transformation applied first. In this case: First rotate the child, then apply the parents scale in y-direction.

If you freeze transfomations, scale will reset to 1 (but the objects will not change shape), and wont affect it's children.

Hard to explain, but if you try to recreate it you will see.

As for your second question, it is a bit more easy. In the attribute editor, a bit down in the first tab, there is a category called limitations. It's rather self explanatary when you have found it. Use that to limit movements. To prohibit movement along one of the axis, right click on the attribute and choose lock attribute.

McWolfe