I don't know about the tax credit for the second car (wish I did), but I can answer your first two questions from experience:
1- Yes, you would need to register your car in order to claim the tax credit. Otherwise to the IRS you are buying it in order to sell it, which is not allowed under either the old rules or the new.
2- You can sell your Venture whenever you like, and it will not affect your tax credit eligibility.
3- I'm not sure I understand this question. Under the old rules you could claim the credit as many times as you liked, even in a single year. Under the new rules, I think there is a limit on the number of times you can claim it in a year, but you are certainly allowed to claim it once each in two successive years. For the used tax credit, there is a limit that the car must be three years old, and can only be claimed once for the used credit in the car's life.