1) There is a proof from the Gemara in Avodah Zarah 65b that one transgresses Lifnei Iver even if no one is Nichshal as a result of the attempt. The Gemara states there that if one knows that wool and linen are present together in a garment but this is not apparent to the onlooker, then he may not sell this garment to a Nochri since it is possible that the Nochri will resell it to a Yisrael. See Tosfos Rid, Avodah Zarah 14a, DH Amar Abaye a'Lifnei, that the reason for this prohibition is because of Lifnei Iver. We see from this that the person who sold the Sha'atnez clothing transgresses Lifnei Iver even though the Nochri might never sell the clothing to the Yisrael, and as a result no Aveirah will result from the sale, but since there is a possibility that the sale will cause a transgression, the person who sold the clothing thereby transgressed Lifnei Iver even if he is not Machshil anyone.

2) There is also a proof from the Gemara in Chulin 64a, where it is stated that one may not sell the egg of a Tereifah chicken to a Nochri, since he might resell it to a Yisrael. Again, even if the Nochri ate it himself, one transgresses Lifnei Iver through the sale. The Chidushei ha'Ramban (Chulin 94a, DH v'Od) writes that the prohibition involved is Lifnei Iver d'Rabanan.

3) There is a certain point in the question that I am unsure about. It seems to be obvious to you that if A gives food to B and tells him that it is kosher but he knows that it is in fact Tereifah, and B did actualy eat the food, that A has transgressed Lifnei Iver. I am not so sure that this is in fact the case. The reason I am wondering about this is because the Rambam in Sefer ha'Mitzvos, Lo Ta'aseh #299, writes that "a stumbling block before the blind" refers to a person whose "bad desires have blinded his intelligent eyes." This suggests that Lifeni Iver is in fact an issue of placing a temptation in front of a person. This is what the Gemara (Avodah Zarah 6b) means when it discusses giving the cup of wine to the Nazir. The Nazir knows that it is a cup of wine, but he is being tempted to drink it and transgress the law relating to a Nazir.

4) In contrast, if a person -- who has been a reliable, observant person up to now -- lies about the food that he is offering, I think it may be that this is not connected to the Sugya of Lifnei Iver. I argue that the person who ate the food is considered as "Anus" -- he has been forced against his will to eat the food. If so, we can refer to the Pischei Teshuvah, Yoreh Deah 29:1, who discusses somebody who ate the meat of animal which later was discovered to have one of the 18 signs of a Tereifah. These 18 Tereifos are rare, and one is not obligated to check that they are not present before eating the meat. If they are later found to be present, the person who ate the meat is considered "Anus" and does not require atonement since he did not willingly commit an Aveirah. I argue that the same would apply to the person who was tricked into eating Tereifah. It would not be considered that he has done an Aveirah, so Lifnei Iver would not apply. The person who bought clothing from a Nochri which afterwards was found to be Sha'atnez, or the person who bought an egg which later was found to be a Tereifah, is different because one has to check in advance what one is buying from a Nochri.

5) A different opinion: I have found a Ritva that seems to say the opposite of what I wrote in my first answer.

The Gemara in Avodah Zarah (end of 63a, 2 lines from the bottom of the page) states that an employer may say to his workers, "Take this coin for your wages," and we are not concerned that the worker might buy fruit of the Shemitah year with the money.

The Ritva there asks that if these workers have not learned the Halachos of Shemitah, why are we not concerned about the prohibition of Lifnei Iver involved when one hands them the money? The Ritva answers that we learn from here that if we do not hand him over the prohibited article itself, and there is a doubt whether or not the person will commit the transgression, this is not considered Lifnei Iver.

6) This matter therefore requires further research. It is possible that there is a dispute between the Rishonim on this question.

Kol Tuv,

Dovid Bloom