The questions are right. As to question 3.b, since the A wins the first and the second backoff race, so A should transmit A3. As to question 5.a, I think the two examples referred are when the link between F and G goes down and when link between A and E goes down. How to choose the hold down period or use certain scheme during hold down period in order to retain the new valid path.
Time is continuous.
No, it is probability problem. If the meaning is infinity, the probability should go to Zero. Here is another question. I think they are using same method. Holddown is to ignore updates for a period of time until link failure news has had a chance to propagate, while the pioson reverse update is to send the negative information.