Sholom Rav.

Acher heard 'Shuvu....chutz me'acher!', because Hash-m wanted him to hear it. Regarding your second question: There are certain sins for which Teshuvah alone does not help, and it is only after Teshuvah, Yom Kipur and death that he goes to Olam ha'Ba. It seems that here, Acher's sins were so bad that Hash-m decreed that his Teshuvah would not help and that Acher had lost his portion in Olam ha'Ba.

Having said that, it seems to me that he was nevertheless obligated to do Teshuvah, since we do Teshuvah because we sinned against Hash-m, not in order to gain Olam ha'Ba. Moreover, if Acher would have done Teshuvah - in spite of the Heavenly Voice, Hash-m would have accepted it. It can be compared to a boy whose father ordered him to remain in his room for a whole day and who cries bitterly and apologizes profusely and pleads with him to forgive him. Do you think that he wouldn't relent?

Be'Virchas Kol Tuv.

Eliezer Chrysler.