I hope you realise that ancient Greek manuscripts did not have punctuation. The comma was added by modern translators to clarify what they took to be the correct interpretation.

The original in Hebrews 10:12 reads: οὗτος δὲ μίαν ὑπὲρ ἁμαρτιῶν προσενέγκας θυσίαν εἰς τὸ διηνεκὲς ἐκάθισεν ἐν δεξιᾷ τοῦ θεοῦ.

From a grammatical point of view εἰς τὸ διηνεκὲς can go either with what precedes it (one sacrifice for sins) or with what follows it (he sat down at the right hand of God). Both interpretations are grammatically correct. I personally think it is methodologically dubious to use texts by other New Testament authors to deduce the meaning intended by the author of Hebrews.