By onen I mean someone who as lost their sibling, and they haven't been buried yet.
Can an onen be called to the Torah as an Oleh? Can an onen read from the Torah?
In response to both your queries:
Even if the only Kohen in shul is an onen, he still doesn't get an Aliyah
On a weekday, this question is a non-starter: the onen is prohibited from making any blessing at all. (Shulchan Aruch Y.D. 241:1)
On Shabbos, when the onen is permitted to attend the synagogue, Rebbi Akiva Eiger (to Y.D. 241:1) believes that he is nevertheless prohibited from getting an aliyah, as he is not allowed to study Torah. (I'm not sure if this is true according to the Dagul MeRvavah to Magen Avraham O.C. 548:8 who writes that an onen is permitted to learn Torah).
Shaarei Efraim (9:111) writes that if an avel was called to the Torah by mistake, then he should accept the aliyah, as refusing would constitute a public display of mourning. I would imagine that R. Margolios would say the same regarding an onen.
Rav Yitzchak Yosef writes (Yalkut Yosef, Bikur Cholim v'Aveilus 7:65) that on Simchas Torah, when everyone in the shul is accustomed to receiving an aliyah, the onen may receive one as well, because avoiding it indicates a public display of mourning, which is prohibited on Yom Tov.