1) Don't open the gifts at the party. If a gift-giver makes a fuss, quietly explain that many guests didn't bring gifts and you don't want to embarrass anyone.
2) I think the bowl of "candy for the road" is a better way to go.
3) Did you give a date to RSVP by? If that date has past, you can call up the non-responders and ask. If they can't give you a straight answer, or a firm date by which they will give you a straight answer, then just say that you're afraid you'll have to put them down as a no. If you can only reach someone's voicemail, leave a message saying that unless you hear from them by X date, you'll understand that they can't make it.
4) I've received a lot of thank you notes from small children that are basically a piece of paper that they have drawn on. The parent writes in somewhere, "Thank you! Susie loves the doll/book/blocks," and signs it. Sometimes they just write "Thank you," in a blank space on the page. As a doting aunt/godmother/friend of the family, that's a perfect thank you in my book.