The gemara explains that the parameters of when we say that one can be a shliach for an aveira to make the sender liable for the action, is either that the shliach is not a bar chiyuva in this prohibition, or that the shliach has no ability to choose not to do it. Both approaches rely on the fact that אין שליח לדבר עבירה is predicated in the premise that rashi points out - דאמרינן ליה דברי הרב ודברי התלמיד דברי מי שומעין ולא היה לו לעשות. Meaning, when the shliach is a bar chiyuva and has the choice to do it or not to do it, the argument can be made that he shouldn't have done it and therefore takes responsibility (the sm"a that i mentioned in the previous post seems to look at it from the perspective of the sender who has an excuse to say that he never expected the shliach to follow through with it). But in a case where the shliach is no a bar chiyuva, there is no reason for him to abstain from doing it, so the sender cannot make the argument that the shliach should not have done it. Certainly, if the shliach is forced to do it and does not make his own choice, he is merely an extension of the arm of the sender so that the sender will be chayev.
Tosafos is bothered by why we consider a yisroel who is acting as a shliach of a kohen to be mekadesh a divorcee, as one who is not a bar chiyuva. Although the yisroel is not included in the prohibition of being mekadesh a gerusha, he is certainly in violation of lifnei iver by assisting the kohen in performing the kiddushin and should be regarded as a bar chiyuva (which would result in ein shliach l'dvar aveira making it that the yisroel is in violation rather than the kohen). Tosafos rejects this concern that we don't determine if the shliach is a bar chiyuva by whether he is doing an issur, rather we determine it by whether the issur that he is doing for the kohen is applicable to him (and there isn't any issur on him to marry a divorcee). The Nodeh B'yehuda (quoted by maharatz chiyus) points out that Tosafos could have simply rejected the issur of lifnei iver causing the shliach to be considered a bar chiyuva, because it is not "two sides of a river" (meaning that the kohen could have done the issur without the yisroel), so it is only an issur d'rabonon of assisting one in doing an aveira, and the mishneh l'melech (hilchos rotzeiach) holds that on an issur d'rabonon we hold יש שליח לדבר עבירה. From the fact that Tosafos doesn't say this implies that Tosafos holds that even on an issur d'rabonon we hold ein shliach l'dvar aveira.
I discussed this issue previously http://hearos.blogspot.com/2009/02/baba-kama-51a-ein-shliach-ldvar-aveira.html. The mishneh l'melech proves that we say ein shliach l'dvar aveira even on an issur d'rabonon from rashi in baba kama who says that we say ein shliach l'dvar aveira on the digging a pit in the public domain even though it is only a rabbinic prohibition of destroying public property. However, the meshech chochma (kedoshim) says that the issur of digging a bor in reshus harabim is the prohibition of lifnei iver, which is d'oraysa, thereby disproving the proof of the mishneh l'melech.