וַיִּרְאוּ אֵת אֱלֹהֵי יִשְׂרָאֵל וְתַחַת רַגְלָיו כְּמַעֲשֵׂה לִבְנַת הַסַּפִּיר וּכְעֶצֶם הַשָּׁמַיִם לָטֹהַר
and they saw the God of Israel: under His feet there was the likeness of a pavement of sapphire, like the very sky for purity.
καὶ εἶδον τὸν τόπον οὗ εἱστήκει ἐκεῖ ὁ θεὸς τοῦ Ισραηλ καὶ τὰ ὑπὸ τοὺς πόδας αὐτοῦ ὡσεὶ ἔργον πλίνθου σαπφείρου καὶ ὥσπερ εἶδος στερεώματος τοῦ οὐρανοῦ τῇ καθαριότητι
And they saw the place where the God of Israel stood; and under his feet was as it were a work of sapphire slabs, and as it were the appearance of the firmament of heaven in its purity.
וַחזוֹ יָת יְקָר אְלָהָא דְיִשׁראל וּתחוֹת כֻורסֵי יְקָרֵיה כְעוֹבָד אַבַן טָבָא שְׁמַיָא לְבָרִירֻו׃
And they saw the Glory of the God of Israel, and under the throne of His glory as the work of a precious stone, and as the face of heaven for its clearness.
The Septuagint tries to avoid anthropomorphism. Like you can see above, the translators did not want to refer to God as visible (like Onkelos). Therefore they reinterpreted the verse, saying that the people saw only the τοπος, the place, where God stood, but not God himself, as the Torah says.
Now we must ask, what does it mean, they saw the place? Was God still there?
Ex. 19:13 says: no hand shall touch him, but he shall be either stoned or shot; beast or man, he shall not live. This verse is talking about the time before the ten commandments, the people should prepare themselves and are not allowed to drive closer to the mountain Sinai, but only when: בִּמְשֹׁךְ הַיֹּבֵל הֵמָּה יַעֲלוּ בָהָר (When the ram's horn sounds a long blast, they may go up on the mountain.) The Septuagint adds to this: καὶ ἡ νεφέλη ἀπέλθῃ ἀπὸ τοῦ ὄρους (and cloud depart from off the mountain, they shall come up on the mountain.) According to this, the cloud in which God appeared was not there anymore, when they went up the mountain later on (24:10), so they saw only an empty place, like the Septuagint says: And they saw the place where the God of Israel stood.
וְעָשׂוּ לִי מִקְדָּשׁ וְשָׁכַנְתִּי בְּתוֹכָם
And let them make Me a sanctuary that I may dwell among them.
καὶ ποιήσεις μοι ἁγίασμα καὶ ὀφθήσομαι ἐν ὑμῖν
thou shalt make me a sanctuary, and I will appear among you
The change here is from שׁכן to ὁράω (fut ind pass 1st sg)
וְשָׁכַנְתִּי בְּתוֹךְ בְּנֵי יִשְׂרָאֵל וְהָיִיתִי לָהֶם לֵאלֹהִים
I will abide among the Israelites, and I will be their God.
καὶ ἐπικληθήσομαι ἐν τοῖς υἱοῖς Ισραηλ καὶ ἔσομαι αὐτῶν θεός
And I will be called upon among the children of Israel, and will be their God.
The change here is from שׁכן to ἐπικληθήσομαι (fut ind pass 1st sg)
עֹלַת תָּמִיד לְדֹרֹתֵיכֶם פֶּתַח אֹהֶל־מוֹעֵד לִפְנֵי יְהוָה אֲשֶׁר אִוָּעֵד לָכֶם שָׁמָּה לְדַבֵּר אֵלֶיךָ שָׁם
a regular burnt offering throughout the generations, at the entrance of the Tent of Meeting before the LORD. For there I will meet with you, and there I will speak with you.
θυσίαν ἐνδελεχισμοῦ εἰς γενεὰς ὑμῶν ἐπὶ θύρας τῆς σκηνῆς τοῦ μαρτυρίου ἔναντι κυρίου ἐν οἷς γνωσθήσομαί σοι ἐκεῖθεν ὥστε λαλῆσαί σοι
a perpetual sacrifice throughout your generations, at the door of the tabernacle of witness before the Lord; wherein I will be make myself known to thee from thence, so as to speak to thee.
The translators changed the verb אִוָּעֵד (I will meet) to אִוָּדַע (I will be make myself known)
יְהוָה אִישׁ מִלְחָמָה יְהוָה שְׁמוֹ
The LORD, the Warrior -- LORD is His name
κύριος συντρίβων πολέμους κύριος ὄνομα αὐτῷ
The Lord bringing wars to nought, the Lord is his name.
וַיְהִי בַדֶּרֶךְ בַּמָּלוֹן וַיִּפְגְּשֵׁהוּ יְהוָה וַיְבַקֵּשׁ הֲמִיתוֹ
At a night encampment on the way, the LORD encountered him and sought to kill him.
ἐγένετο δὲ ἐν τῇ ὁδῷ ἐν τῷ καταλύματι συνήντησεν αὐτῷ ἄγγελος κυρίου καὶ ἐζήτει αὐτὸν ἀποκτεῖναι
And it came to pass that the angel of the Lord met him by the way in the inn, and sought to slay him.
Not God himself kills, but through an angel. See Onkelos: וַהְוָה בְאוֹרחָא בְבֵית מְבָתָא וְעָרַע בֵיה מַלאְכָא דַיוי וּבעָא לְמִקטְלֵיה׃ (And it was in the way, at the place of lodging, that the Angel of the Lord met him, and sought to kill him)
וַיֹּאמֶר אֲנִי אַעֲבִיר כָּל־טוּבִי עַל־פָּנֶיךָ וְקָרָאתִי בְשֵׁם יְהוָה לְפָנֶיךָ וְחַנֹּתִי אֶת־אֲשֶׁר אָחֹן וְרִחַמְתִּי אֶת־אֲשֶׁר אֲרַחֵם
And He answered, "I will make all My goodness pass before you, and I will proclaim before you the name LORD, and the grace that I grant and the compassion that I show.
καὶ εἶπεν ἐγὼ παρελεύσομαι πρότερός σου τῇ δόξῃ μου καὶ καλέσω ἐπὶ τῷ ὀνόματί μου κύριος ἐναντίον σου καὶ ἐλεήσω ὃν ἂν ἐλεῶ καὶ οἰκτιρήσω ὃν ἂν οἰκτίρω
And God said, I will pass by before thee with my glory, and I will call by my name, the Lord, before thee; and I will have mercy on whom I will have mercy, and will have pity on whom I will have pity.
This verse is amazing. God will pass his by his δόξα before Moshe, which is his nature so to speak. We know from verse 20 (you cannot see My face, for man may not see Me and live.) that Moshe cannot see the δόξα, therefore God reveals himself as the one who has mercy and pity, the only way to perceive God. (Frankel)
וְדִבֶּר יְהוָה אֶל־מֹשֶׁה פָּנִים אֶל־פָּנִים כַּאֲשֶׁר יְדַבֵּר אִישׁ אֶל־רֵעֵהוּ
The LORD would speak to Moses face to face, as one man speaks to another
καὶ ἐλάλησεν κύριος πρὸς Μωυσῆν ἐνώπιος ἐνωπίῳ ὡς εἴ τις λαλήσει πρὸς τὸν ἑαυτοῦ φίλον
And the Lord spoke to Moses face to face, as if one should speak to his friend
At first sight there is no difference between the Septuagint and the Masoretic text. On closer inspection, when we consider verse 20 (you cannot see My face, for man may not see Me and live), we have to ask why the Septuagint in 20 uses the word πρόσωπον for פני (face) but in verse 11 ἐνώπιος, which is a synonym. Maybe the phrase ἐνώπιος ἐνωπίῳ (face to face) is more abstract than πρόσωπον and could mean: from person to person? See Onkelos: וּממַלֵיל יוי עִם מֹשַׁה מַמלַל עִם מַמלַל כְמָא דִימַלֵיל גֻברָא עִם חַברֵיה (And the Lord spake with Mosheh word with word, as a man speaketh with his companion)