Gonna rearrange your questions a bit, Stephen, as some are germaine to others preceding them:
Has it been put forward concerning this phrase that the LXX rendering is more likely due to a specific ignorance in geography, or to an avoidance of mentioning a place following the Judea - Sameria schism.
The LXX Genesis translator prefers to translate rather than transliterate place-names that you and I are probably more familiar with, such as in Gen. 16:13 'El-Roi
becomes ὁ θεὸς ὁ ἐφιδών με; Gen. 25:11 Beer-Lahai-Roi
becomes τὸ φρέαρ τῆς ὁράσεως. However these aren't technically proper nouns, and the LXX translator does like to transliterate the proper nouns in Genesis.
Stephen Hughes wrote:
I'm not familiar enough with Hebrew to go any further with this question about this variation between the MT and the LXX Vorlage. The specific things I find myself wondering about are these:
Genesis 50:13 (MT) wrote:וַיִּשְׂא֨וּ אֹת֤וֹ בָנָיו֙ אַ֣רְצָה כְּנַ֔עַן וַיִּקְבְּר֣וּ אֹת֔וֹ בִּמְעָרַ֖ת שְׂדֵ֣ה הַמַּכְפֵּלָ֑ה אֲשֶׁ֣ר קָנָה֩ אַבְרָהָ֨ם אֶת־ הַשָּׂדֶ֜ה לַאֲחֻזַּת־ קֶ֗בֶר מֵאֵ֛ת עֶפְרֹ֥ן הַחִתִּ֖י עַל־ פְּנֵ֥י מַמְרֵֽא׃
Genesis 50:13 (LXX) wrote:καὶ ἀνέλαβον αὐτὸν οἱ υἱοὶ αὐτοῦ εἰς γῆν Χαναὰν καὶ ἔθαψαν αὐτὸν εἰς τὸ σπήλαιον τὸ διπλοῦν, ὃ ἐκτήσατο ῾Αβραὰμ τὸ σπήλαιον ἐν κτήσει μνημείου παρὰ ᾿Εφρὼν τοῦ Χετταίου, κατέναντι Μαμβρῆ.
My general question is whether the LXX phrase τὸ σπήλαιον τὸ διπλοῦν "the double cave", a possible and valid rendering of the Hebrew מַכְפֵּלָה (if the pointing were different)?
As noted above, the LXX Genesis translator preferred to transliterate rather than translate proper nouns, so a question we should ask here actually is, "Is Machpelah a proper-noun?"
In Hebrew, proper nouns aren't prefixed with the definite article, yet here in Genesis 50:13 we have הַמַּכְפֵּלָ֑ה literally "the Machpelah" - this would therefore indicate that thinking of Machpelah as a location name is in error.
Looking at the other places where הַמַּכְפֵּלָ֑ה appears, Gen 23:9, 17, 19; 25:9; 49:30; bar Gen 23:17, it always has the definite article prefixed - in Gen 23:17, the definite article has been technically "swallowed" by the prefixed preposition ב.
Checking this places in the LXX, we have:
Gen 23:9: τὸ σπήλαιον τὸ διπλοῦν
Gen 23:17: ἐν τῷ διπλῷ σπηλαίῳ
Gen 23:19: ἐν τῷ σπηλαίῳ τοῦ ἀγροῦ τῷ διπλῷ
Gen 25:9: τὸ σπήλαιον τὸ διπλοῦν
Gen 49:30: ἐν τῷ σπηλαίῳ τῷ διπλῷ
As you can see, the LXX translator has taken הַמַּכְפֵּלָ֑ה to be an adjective each time, always preceded by the definite article. I would likely suppose that the definite article before the word is what has influenced the translator to not take מַּכְפֵּלָ֑ה as a proper noun.
What would the correct form of the feminine participle (perhaps מוכפלה) from כָּפַל "double" (including the pointing) need to be to have the meaning "doubled"? Would the addition of the waw in the participial form, and its absence in the מַכְפֵּלָה be enough to let us understand what might have been going through the LXX translators' mind while reading of the text - is the spelling of participles flexible in this (waw) regard?
Afraid I don't work with pointed Hebrew text to give you much help on this.
Seeing as this is not translated as δεδιπλωμένον "doubled" in the LXX, but as διπλοῦν "double",
See above regarding the definite article.
let me ask a general question: Are Hebrew participles usually rendered as Greek participles? In other words, if this διπλοῦν is in fact a rendering of of the Hebrew participial form, is the choice of a Greek adjective noteworthy or commonplace?
No, not really, and it is quite commonplace for the LXX to translate nouns as verbs, participles as nouns, verbs as participles, and any and all inbetween.
Though again, in this case, see above regarding the definite article.
Is it reasonable to assume that the שְׂדֵ֣ה of the MT was not in the LXX Vorlage?
From the verses quoted above, I would say it's highly likely that שְׂדֵ֣ה was not in the LXX Vorlage. If it had been, it would've looked like 23:19 above (which is almost the exact same wording as 50:13 in Hebrew). Also, it looks like the LXX Vorlage at 49:30 was also missing the שְׂדֵ֣ה, which is there in the MT.
Hopefully that helps