Regrettably, it happens too often that a Bible passage turns out hard to understand on account of the fact that many Hebrew prepositions – originally univocal terms – were reduced, over the centuries, to single letter. Isa 59:20 is just a typical case of this kind.
All the trouble is focused on the term לציון, that is made up of two parts: ‘Zion’ [ציון], preceded by the hyper-synthetic proposition L- [ל].
Now, if you search for the meaning of - ל in the Hebrew lexicons more probably you will be astonished. Why? Well, the Davidson lexicon lists twelve different meanings linked with L- [ל]. Gesenius assigned fourteen meanings to it, at least. John Parkhurst has a list with 22 different meanings of it. Koehler&Baumgartner: 26 meanings. And – sincerely - I have had enough to count all the different meanings listed by Schöckel…
In these cases, how we can unravel the dilemma? Since this is not a linguistic site, I will answer simply, through textual criticism. In this specific case, we have to consider the witnessing of the context (in particular, the mention Paul made in Romans of this passage), and of the ancient translations.
In the Septuagint (LXX) the verse is: “καὶ ἥξει ἕνεκεν Σιων ὁ ῥυόμενος καὶ ἀποστρέψει ἀσεβείας ἀπὸ Ιακωβ.”, that translated by Brenton is: “And the deliverer shall come for Sion's sake, and shall turn away ungodliness from Jacob”.
As you note, here ἕνεκεν is a pivotal term. It means ‘on account of’, ‘for’ (I don’t know where ‘User 33515’ has found his reading of ‘Septuagint…). So, for the Jewish translators of the Septuagint (if they translated by a similar Hebrew text that we have to disposition), the L- [ל] had the meaning of ‘on account of (Zion)’, and not – merely – ‘to (Zion)”.
Someone may think that the Paul’s paraphrase of this Isaiah passage – using the preposition εκ – contradicts this conclusion. But, also in this case, wait a moment. One of the various meanings of the Greek preposition εκ is – again – ‘on account of, ‘for’ (you may examine, for some examples, this usage of εκ by Sophokles, Plato, and Plutarch. If you have need a more detailed references from this works I am able to list them).
So, the sense of Isa 59:20 is the one and the same of Rom 11:26. There’s no change of meaning.
Also, a number of Bible translation reached the same conclusion (bold is mine).
“Und ein Erlöser wird kommen für [‘for the sake of’] Zion und für die, welche in Jakob von der Übertretung umkehren, spricht Jahwe”. (Elberfelder Bibel)
“E un redentore verrà per [‘for’] Sion e per quelli di Giacobbe che si convertiranno dalla loro rivolta, dice l'Eterno.” (Riveduta-Luzzi)
“E verrà per [‘for’] Sion un redentore […]” (Bonaventura Mariani)
“[…] for Zion will come a redeemer” (NJB)
“[…] come redentore verrà per [‘for’] Sion” (La Civiltà Cattolica-Piemme; in a similar manner also the TOB)
“Ma per [‘for’] Sion viene quale redentore […]” (Concordata).
Very interestingly, this manner to translate - both in Hebrew and Greek - enhances the loyalty and the care of the Creator towards men, since He is ready to deliver peoples from their errors.
I hope these information will be useful to you.