Questions from a noob

Brand new player still learning.

1. Does the way a model is facing impact it's ability to assist or defend a block? For example are you more likely to be knocked down if you're tackled from behind?

2. What do the -1, -2, and -3 numbers indicate which are placed around the opponent's team? Are they a reduction in rolls, AV, or something else? If they reduce rolls is it only to tackle or is it also to cause injury?

3. What does the +1 strength do when there are adjacent players around? Does it increase chance of successfully blocking or does it increase chance of causing injury or does it do both?