Qurbaani

Q: Why qurbaani is performed in odd numbers rather than even, such as seven shares for a cow and one share for a goat? Why not six shares for a cow or two shares for a goat?

Q: Last year qurbaani was waajib on me. However, I was not aware of it. Nevertheless, my father has performed qurbaani for me with his money. Will it suffice for my waajib Qurbaani. If not, do I have to do to rectify my mistake?

Q: When calculating your zakaat, you take into consideration the money that is owed to you. 
Will this also apply on the days of qurbaani in determining if it is incumbent upon one to slaughter or not?