ᾖ μεθ’ ὑμῶν εἰς τὸν αἰῶνα,I shouldn't have to, if you are as competent with your Greek as you think you are. Both prepositional phrases are used adverbially, and modify the verb.
How would that come out in English? “He will be with us, he will be unto eternity?” Nonsense.
The object of the preposition μεθ is ὑμῶν , and εἰς τὸν αἰῶνα is modifying ὑμῶν. You are just wrong.