how to prevent pain in blocking hockey shots in the feet/ankles?

Some skates have a thicker construction than others. High-end skates tend to be lighter, and generally a little bit slimmer, which means more direct contact with your foot. It you get hit with a slapshot in the foot, it's going to hurt, the ankle is especially painful. The best way to avoid such contact would be to stay square to the shooter as much as possible. If you are facing the shot, it will hit the toecap of the skate, which is the toughest and hardest part, meaning the least painful. Other that that, there isn't a whole lot you can do.

well you're right when you say ice time is the greatest cure for that. getting your back to adapt to the skating motions will help hit prevent such lower back pain. usually I have it at the beginning of the season, but its gone by the second game. there are many exercises to work out your back, simply ask a trainer at your gym.

You should stay up, rather than slide to block shots, you would have a better chance to avoid being hit. Many defensemen stand in position, and only to move at last minute, to avoid getting hurt.

If in goal protection on the sides (the plastic cowling part) should eliminate impact although you might have outgrown your pads. If skating out consider ankle guards.

Face the shot without turning your feet sideways.

Well.... I can't really tell you anything. Taking a puck to the ankle is gonna hurt that's how it is.

Stay on the bench.