Do Non-Jews have the rule of Safek D'Oraisa L'Chumra (if one is unsure about whether a Torah prohibition exists, He must assume the more strict behavior), concerning the 7 mitzvos of B'nei Noach or does this rule apply only to Jews?
I saw this discussed in Ateres Shmuel (by Rav Mordechai Rosenbaum) to Kesubos 15b ד"ה אם רוב עכו"ם .
The gemarra there says an abandoned child that is found in a city of mostly non Jews is treated as a non Jew, to the extent that we can directly feed him non kosher food (Rashi). The Ateres Shmuel asks why didn't Rashi just say the child can feed himself non kosher?
He says quoting Rav Avraham Gurwicz that even if the city was 50% Jews 50% non Jews, the child can be lenient and eat non kosher. This is because saffeik doraisa lechumra according to the Rambam is deRabannan, and only a definite Jew has to listen to the Rabbis. Not sure his source for that last part. But we see from here that a definite non Jew doesn't have to worry about saffeik doraisa lechumra. So for example, if he has a saffeik if his food is maybe eiver min hachai, he can eat it (assuming the saffeik is 50-50).
The Ateres Shmuel considers saying that even according to those who say saffeik doraisa lechumra is a doraisa, this questionable non Jew can still be lenient. However, he brings the Rash's opinion that if the city was 50% Jews 50% non Jews, the child can't eat non kosher. He presumably holds saffeik doraisa lechumra is a doraisa. We would then have to conclude either (1) a non Jew has to follow saffeik doraisa lechumra, and in the above case they'd have to worry the food is eiver min hachai, or (2) at least a questionable non Jew has to be stringent, but maybe a definite non Jew not.
Now this assumes the saffeik is 50-50. If there is a majority saying it is muttar, there is a discussion in Achronim if non Jews can rely on the rule of rov (majority). If yes, no problem. If no, then your question will also apply.
The Rambam Malachim 9 says that since Bnei ketura and bnei Yishmael intermarried, they're both obligated in circumcision misafek. We see you say safek deoraisa by non jews. It's also very logical to say so because the Rashba said that the Rambam said his source for safek Deoraisa likula is learned from the fact a safek mamzer is permmited likhal. It's a derivation that would only apply to jews. Without this derivation, logic says safek deoraisa is lichumra. Everyone agrees logic says safek deoraisa is lichumra, therefore it should apply to non jews as well.