Finish the entire pack of Nordette.

Yes, you can stop in the middle of one pack. So long as you start the new birth control right away (no break in between), you should be fully protected. However, you might experience spotting or breakthrough bleeding.

Ideally yes, you should wait until the end of one pack of birth control before switching, but given your situation, I'd say to just either get another pack of Yasmin and finish it, or else get the Nordette and start that right away. Whatever you do, time is running out. If you've missed two or more pills in a row by this point, then whenever you get the pack of pills, be it Yasmin or Nordette, take ALL 21 pills, and use back up for seven days, as missing two or more pills in a row greatly lowers the effectiveness of the pill.

If you've already gotten a new pack of Yasmin, and are taking that, then I'd recommend finishing that pack and THEN starting the Nordette.

Again, the longer you leave getting a new pack, the worse it is, and the more messed up your cycle will be.