Can a non Jew be buried in a Jewish Cemetery under any circumstances?
Gittin 61 Says we bury the dead of non-Jews with dead of Jews. (קוברים מתי עכו"ם עם מתי ישראל). This is mipnei darkei shalom
Rashi there comments that the gemarra shouldn't be understood as "with" literally, but "also" like we bury our own, when we find them together.
Rambam brings gemarra down as is.
Tur brings down gemarra and Rashi. Beis Yosef notes the Ra"n that "finding together" in Rashi is lav davka and we bury them even if we find only a non-Jew. The Ra"n however adds, that we do not bury two people on different levels next to eachother (such as a tzaddik by a rasha, even a tzaddik by a chassid) (SA & Tur YD 382) so this should hold the same reasoning (He uses the phrase כל שכן, usually meaning it's a kal v'chomer).
Bach YD 151 is medayek from the Rambam that it would be permissable (possibly only referring to the case where they were found together - this point comes and goes and I couldn't understand were it was standing when he reached this part).
According to this source (son of Rabbi Y.M. Lau) different chief rabbis discussed the matter, ruling to forbid it. I've heard Rav Ovadia gave a similar psak.
Here it says Rav Goren allowed it.
Finally Rav Dov Lior answered that if a non-Jew was found buried in a Jewish cemetery - he should be moved. (non-Jewish grave found in Jewish cemetery)