1. The only action that Frozen models may take is to attempt to break free. That being the case, it is implied that, while Frozen, models cannot be attacked, since they are "frozen in a block of ice".
So, I personally would rule it as follows: while frozen, models cannot normally be attacked. If you choose to do so, you are attacking through the ice they are frozen in, and thus the Freeze is dispelled.
2. Yes, undead can be frozen....bones do break, after all....and get quite brittle in extreme cold....