The editor suggested that I improve my post with more documentation. So, the following answer is edited to provide substantiation for my claims:
You ask a really good question.
The first fact we have to realize is that there are words in the Hebrew that are hard to figure out. The verb translated as "strike/bruise/crush" in the Hebrew is one of those. The precise meaning of the verb is not precisely known. Here's the description that the Hebrew and Aramaic Lexicon of the OT gives us:
- it should be noted that in general for the translation of this verb we follow Gesenius-Buhl Handw., Zorell Lexicon, and König Wb. 490a, in supposing that there is only one verbal root שׁוף; so also von Soden UF 13 (1981) 160f ( = Bibel und Alter Orient, BZAW 162 (1985) 200f); the same verb is used twice also in Sept. for Gn 315 τηρήσει, τηρήσεις :: KBL which takes the two forms as coming from separate verbs: I שׁוף to bruise someone’s head Gn 315; and II שׁוף to snap at, snatch (? a by-form of → שׁאף), to snatch someone’s heel Gn 315; to snatch someone (with acc. of the person) Jb 917; d) identical or comparable verbs in other Semitic languages are attested in MHeb., DSS (Kuhn Konkordanz 219), to rub, rub away; so also JArm. I שׁוף; II to blow; Syr. šāp to stroke, spread;
HALOT, s.v. “שׁוף,” 4:1446.
Likewise, BDB gives the following:
7779 † [שׁוּף] vb. bruise (NH id., Aram. שׁוּף, שְׁפַף, ܫܳܦ rub off, away, grind (Ex 32:20 for טָחַן); BDB, s.v. “שׁוּף,” 1003.
Likewise the NIDOTTE:
OT 1. The vb. שׁוּף occurs twice in Gen 3:15, the so-called Protevangelium, the first utterance of the gospel after the Fall into sin. Yahweh God speaks to the serpent:
And I will put enmity between you and the woman,
and between your offspring and hers;
he will crush your head,
and you will crush (or snap at?) his heel.
The nature of the evidence from the Targums, the LXX, and Vg. is such that little help is forthcoming for translating שׁוּף. As far as the context is concerned, two observations can be made. In favor of two different roots and translating “crush” and “snap at” respectively is that “the two actions are different, corresponding to the different bodily forms of the parties” (C. Westermann, Genesis 1–11, 1974, [ET 1984], 260). On the other hand, it is understandable that the majority of modern scholars prefer keeping the sense the same in both instances (see G. J. Wenham, Genesis 1–15, WBC, 1987, 80), because of the reciprocative nature of the passage. Furthermore, the sense of “crush” or “batter” also fits the two other occurrences of this vb. in Job 9:17 and Ps 139:11. (Cf. G. R. Driver in JTS 30, 1928–29, 375–77 and A. Guillaume in JSS 9, 1964, 286–88.) Cornelis Van Dam, NIDOTTE, s.v. “שׁ,” 4:67.
So the discussion/debate is what root the word is from. And as a result, we have different options:
Whenever we find difficult words to understand in the source language (Hebrew), we find translators having to choose a word in the receptor language (Greek), even if they do not know precisely the meaning of the word they are translating.
So what does ⲧⲏⲣⲉⲱ mean here? Usually it means to hold onto/keep/preserve.
Cf. BDAG:
τηρέω impf. ἐτήρουν, 3 pl. ἐτήρουν and ἐτήρουσαν AcPl Ha 8, 11 and 13; fut. τηρήσω; 1 aor. ἐτήρησα; pf. τετήρηκα, 3 pl. τετήρηκαν J 17:6 (B-D-F §83, 1; W-S. §13, 15; Mlt. 52f; Mlt-H. 221). Pass.: impf. ἐτηρούμην; 1 aor. ἐτηρήθην; pf. τετήρημαι (Pind., Thu.+)
to retain in custody, keep watch over, guard τινά, τὶ someone, someth. a prisoner (Thu. 4, 30, 4) Mt 27:36, 54; Ac 16:23; a building (s. PPetr II, 37, 1, 19 [III BC] τηρεῖν τὸ χῶμα; PFlor 388, 32; 1 Macc 4:61; 6:50) Hs 9, 6, 2; 9, 7, 3. Pass. (Jos., Ant. 14, 366) Πέτρος ἐτηρεῖτο ἐν τῇ φυλακῇ Ac 12:5. Cp. 24:23; 25:4, 21b. τηρεῖν τὴν φυλακὴν guard the jail 12:6. ὅπου οἰ κεκλεισμένοι τηροῦνται AcPl Ha 3, 20. Abs. (keep) watch (PSI 165, 4; 168, 9; 1 Esdr 4:11; 2 Esdr 8:29) MPol 17:2. οἱ τηροῦντες the guards (SSol 3:3) Mt 28:4.
to cause a state, condition, or activity to continue, keep, hold, reserve, preserve someone or someth. BDAG, s.v. “τηρέω,” 1002.
Cf. BrillDAG:
τηρέω, contr. [see τημελέω ?] impf. ἐτήρουν, mid. pass. ἐτηρούμην || fut. τηρήσω, mid. (pass. signf.) τηρήσομαι || aor. ἐτήρησα, mid. ἐτηρησάμην Orig. (PG 13.25.5) || pf. τετήρηκα, mid. pass. τετήρημαι || ppf. (ἐ)τετηρήκειν, mid. pass. (ἐ)τετηρήμην || aor. pass. ἐτηρήθην || fut. pass. τηρηθήσομαι
act.
a. to watch over, protect Hom. 2.142 Pind. P. 2.88 Aristoph. Ran. 1516 Orph. H. 64.4; subst. ptc. οἱ τηροῦντες the guards NT Matt. 28.4 | rar. of pers. Aristoph. Nub. 579; ▸ τινὰ τ. ἀπό τινος to protect s.o. from s.o. VT Prov. 7.5 || to pay attention, take care ▸ ὅπως that Aristot. Pol. 1309b 16 | ▸ ὅπως μή lest Demosth. 18.276 | ▸ μή lest Aristoph. Th. 580 Plat. Theaet. 169c || to revere, venerate: ἕνα … θεόν a single god Orig. HIer. 18.9
b. to observe, keep an eye on, pay attention to ▸ with acc. Thuc. 4.60.1 Aristoph. Eq. 1145, Ve. 364 Men. Epit. 375 Sor. 1.41; εἶμι τηρήσουσ’ ὅ τι καὶ δράσει I will keep watch to see what he will do Aristoph. Ec. 946; ἀλλὰ ἄλλοτε ἄλλῃ τὰς αἶγας ἔνεμεν ἐκεῖνον μὲν φεύγων Χλόην δὲ τηρῶν he was constantly pasturing the goats in different places, keeping away from him, and keeping an eye on Chloe Long. 4.12.3 | ▸ with inf. to watch out for Thuc. 4.26.7 The Brill Dictionary of Ancient Greek, s.v. “Τηρευς,” 2115.
Cf. Lidell Scott:
τηρέω, f. ήσω, (τῆρος):—to watch over, protect, guard, Pind., Ar.:—Pass. to be constantly guarded, Thuc.; f. med. τηρήσομαι in pass. sense, Id.
- to take care that. . , Arist., Ar., Plat.
II. to give heed to, watch narrowly, observe, Ar.; τὰς ἁμαρτίας Thuc.
- to watch for, c. acc., Soph., Ar.; παραστείχοντα τηρήσας having watched for him as he was passing by, Soph.
- absol. to watch, keep watch, Arist.:—c. inf. to watch or look out, so as to. . , Thuc.
III. to observe or keep an engagement, Isocr., etc.; τ. εἰρήνην Dem. Hence τήρησις An Intermediate Greek-English Lexicon, s.v. “τηρέω,” paragraph 56180.
Here it's difficult to tell why they chose that verb. The LXX rendering has no understandable connection to the verb in the Hebrew. But, for the sake of comparison, let's look at the other versions:
“Ipsa tibi servabit caput, et tu servabis eius calcaneum” (Gen. 3:15 V-LATINA)
Translation: "She will guard/preserve your head. And you will guard/preserve his heel." Here it's apparent that the Vetus Latina derived its translation from the LXX, not from the Hebrew source.
What does Jerome's Vulgate yield? Let's look:
“ipsa conteret caput tuum et tu insidiaberis calcaneo eius” (Gen. 3:15 VULG-T)
Translation: "She will grind/pound your head and you will lie in ambush for his heel." Jerome is clearly working with the Hebrew text. And one can see that in his closer correspondence to the "grind/strike/pound" meaning.
How about the Syriac Peshitta?:
”ܗ݂ܘ ܢܕܘܫ ܪܫܟ. ܘܐܢܬ ܬܡܚܝܘܗܝ ܒܥܩܒܗ܀“ (Gen. 3:15 PESHOT-T)
Translation: "He will trample your head. And you will strike his heel." Here too, we see a close correspondence between the Hebrew text and the target language, Syriac.
So, when you stack up the evidence from the versional support, you see the LXX go in its own direction (with the Vetus Latina following it, since it's clear in this section that it was translated from the LXX, not from the Hebrew). But the other versions (Vulgate and Peshitta) closely following the Masoretic Text.
So, to finally answer your question:
What are some good resources to use to understand the LXX writers' decisions?
There are none because, in this instance, nobody has a clue why the LXX translators went with the verb, ⲧⲏⲣⲉⲱ, in their translation.
All translations provided above are my own