Rachel stealing the terafim: afrushei m'siursa by a ben noach

I ended off before Shabbos with the following question: the Meshech Chochma (and the Ohr haChaim learns this way as well) explains that when Ya’akov said that whoever stole Lavan’s terafim should die, it was not just a kelala, but it was a psak din -- a ben noach is chayav misa for gezel.  If so, how could Rachel have done it?

My initial thought was that that Rachel was acting to prevent Lavan from doing an issur of avodah zarah (this assumes that terafim were an avodah zarah, but not all the meforshim accept that) and therefore she had license to do what she could l’afrushei m’isura.  What I wasn't sure about was whether there is a concept of afrushei m’isura by a ben noach.  The gemara says explicitly that that there is an issur of lifnei iveir to trip up a ben noach and cause him/her to violate an issur, but lifnei iveir only applies where you are the sole means by which the individual in question has the means to do the issur, e.g. you are the only avodah zarah salesman in town.  If the aku"m has other means of getting his idols, your selling him one is not lifnei iveir.  It would, however, pose a problem of the issur derabbanan that says you should try l'afrushei m'isura.  Shach in Y.D. 151:6 says that there is no requirement of being afrushei m'isura by an aku"m, but the Gilyon Maharasha on the spot sends you to a MG”A that disagrees.  (I don't know whether this hinges on whether the derabbanan of afrushei m'isura is an extension of lifnei iveir or a separate din.)

That being said, I’m not sure the cases are comparable.  The Shach is speaking about avoiding doing something which would enable an aku”m to do issurim, like selling him an idol.  The case of Rachel stealing the terafim is a case of her violating an issur misa to prevent another ben noach from a different issur misa.

I saw the following quoted in the name or R’ Chaim: when Ya’akov originally paskened that whoever stole the terafim was chayav misa, he assumed that one of the servants, a ben noach, was the thief.  The pasuk continues and tells us that Ya'akov got it wrong because, “V’lo yada ki Rachel ganavasam,” and Rachel has a din of a yisrael, not ben noach, and therefore there is no chiyuv misa.
 
Yes, saying it was an aveira lishma solves everything, but you can use that to explain away any apparent misdeed on the part of on of the Avos or Imahos.