The problem with the first two renderings is that the subjunctive can be explained by the particle / preposition before them. So really only the subjunctive in verse 15 should be treated as hortatory. This can be seen by Wallace categorising hortatory subjunctives as being in independent clauses
Verse 13 is explained by μεχρι and the fact that attaining is not a given which would more naturally be in the indicative. Given that, a hortatory subjunctive here is unlikely. The wider context helps here
Jesus gave some as apostles etc, verse 12 gives the reason which is the preparation of the saints for / resulting in good works, for the building up of the church until all attain the unity of faith. To say that it is “for the building up of the church until let us all” doesn’t make any sense. The subjunctive here is also in a dependent clause (I think)Καὶ αὐτὸς ⸀ἔδωκεν τοὺς μὲν ἀποστόλους, τοὺς δὲ προφήτας, τοὺς δὲ εὐαγγελιστάς, τοὺς δὲ ποιμένας καὶ διδασκάλους, 12 πρὸς τὸν καταρτισμὸν τῶν ἁγίων εἰς ἔργον διακονίας, εἰς οἰκοδομὴν τοῦ σώματος τοῦ Χριστοῦ, μέχρι καταντήσωμεν οἱ πάντες εἰς τὴν ἑνότητα τῆς πίστεως καὶ τῆς ἐπιγνώσεως ⸋τοῦ υἱοῦ⸌ τοῦ θεοῦ, εἰς ἄνδρα τέλειον, εἰς μέτρον ἡλικίας τοῦ πληρώματος τοῦ Χριστοῦ,
Ινα takes the subjunctive as the mood of the verb in the clause it introduces. This explains the subjunctive there, a hortatory subjunctive wouldn’t be with this particle as far as I know. Ινα itself specifies the beliefs/desires as of the person referred to, this could be rendered as “in order that” although not necessary. So the text for verse 14 could specify the reason in this instance for the preceding, regardless - the author believes we should no longer be infants etc but it isn’t expressed as an encouragement to action. Again, this is in a dependent clause
4:13-15 MGNT] (13) μέχρι καταντήσωμεν οἱ πάντες εἰς τὴν ἑνότητα τῆς πίστεως καὶ τῆς ἐπιγνώσεως τοῦ υἱοῦ τοῦ θεοῦ εἰς ἄνδρα τέλειον εἰς μέτρον ἡλικίας τοῦ πληρώματος τοῦ Χριστοῦ (14) ἵνα μηκέτι ὦμεν νήπιοι κλυδωνιζόμενοι καὶ περιφερόμενοι παντὶ ἀνέμῳ τῆς διδασκαλίας ἐν τῇ κυβείᾳ τῶν ἀνθρώπων ἐν πανουργίᾳ πρὸς τὴν μεθοδείαν τῆς πλάνης (15) ἀληθεύοντες δὲ ἐν ἀγάπῃ αὐξήσωμεν εἰς αὐτὸν τὰ πάντα ὅς ἐστιν ἡ κεφαλή Χριστός
I think for 15 we have a nominative plural participle which agrees with the subjunctive following it. Your paste missed the comma that NA28 has, which marks this as beginning a new clause. So here the verb isn’t in a dependent clause but is an independent clause and therefore can be a hortatory