1 and 3 can't work, as 1 would leave control of the private key with the vendor. 3 defeats the purpose of having a denomination on the note.
1 is likely to work more for social reasons, not technical reasons. Sure, the vendor could swipe you for 0.23 BTC, if they want to leave a bad taste in your mouth. They could also double-charge your VISA today, but by and large they don't. Now, you
do want to watch out for dishonest clerks, because they might have less of a stake and would be more interested in scamming you. But the same way people know to do due diligence now to protect themselves from hacks of online wallets, so can people learn to avoid getting scammed by clerks (e.g. don't let the clerk snap photos of your change paper, or use method #2 or #3 whenever possible).
3, given the notes are meant to be disposable, simply crossing out the denomination and writing a new one isn't all that outrageous. Or putting a small tear in the note to signal it has been partially spent. Not all notes have to be printed pre-denominated. Notice on my sample notes I simply hand-wrote 1 BTC on them and I don't consider this a terrible flaw.
I personally LOVE this idea casascius! I want to see BTC accepted in more physical stores/restaurants and I think this is the way to do it. I like #1. If you are concerned about the private key, just take your receipt to your table/car and scan it with your phone and move to your own address.