εῑμι has future meaning but what about ἁπειμι, ἕξειμι etc?
Yes, ἄπειμι (ibo) serve as the future of ἀπέρχομαι, ἔξειμι (ibo) as the future of ἐξέρχομαι
Regularly in Attic compound+εἶμι has future sense. But LSJ points out some exceptions: “the pres. sense is sts. found … in compds. (προσ-) A.Eu.242, (ἐπ-) Th.4.61, (συν-) Str.3.2.2.”