David Lim wrote:And by the way, Ken, can you explain why the LXX used "εσται εαν ...", instead of "εσται οταν" or something, since it is a certainty that his days will indeed be fulfilled?
Oops. I thought you were speaking of 7:14. It's 7:12 you mean.
Well, I don't have an explanation, but I can say a few things that might be relevant.
KY is translated as εαν 4 or 5 times more commonly than as οταν. ἔσται ἐὰν is quite unusual, and as you noted, unexpected in this context. Some parallel expressions:
Judges 21:22 καὶ ἔσται ὅταν ἔλθωσιν οἱ πατέρες αὐτῶν ἢ οἱ ἀδελφοὶ αὐτῶν κρίνεσθαι πρὸς ὑμᾶς
Exodus 12:16 καὶ ἔσται ἐὰν λέγωσιν πρὸς ὑμᾶς υἱοὶ ὑμῶν Τίς ἡ λατρία αὕτη;
The translator misread the tetragrammaton YHWH for WHYH. This is a very easy mistake to make. That's why the versification differs between the Hebrew and Greek. The Hebrew YHWH belongs with the preceding verse, but the Greek και εσται belongs with the following verse.