יַהְוֶה (English transliteration: Yahveh/ Yahweh) would theoretically be the root הי"ה conjugated in binyan Hif'il, 3rd person, masculine gender, singular number, imperfect tense. It would then mean, "he/ it causes to be." This particular binyan doesn't actually exist, or is not used, in biblical or modern Hebrew (not to my knowledge at least). But, considering its potential meaning, that would be expected since only God truly causes anything to be (to exist).
As for יְהוָֹה (English transliteration: Yehovah/ Yehowah), it's a documented fact that the Masoretes placed the nikkud of אֲדֹנָי under the letters יהוה. The only difference is that the chataf patach under the א of אֲדֹנָי becomes a shva under the י of יְהוָֹה since an א can never be pointed with a shva.
As for the sound /yahu/ found at the end of many names, you should also notice that those same names sometimes end in /yah/. For example, compare Jer. 28:5 /Yirmeyah/ (יִרְמְיָה) v. Jer. 7:1 /Yirmeyahu/ (יִרְמְיָהוּ). Why, then, is the -וּ often written? At this time, I'm not sure. I'd have to dig through Gesenius for a reason.
Also, note that we find the name יָהּ (English transliteration: Yah) occurring forty-nine times in the Masoretic text.
Finally, you wouldn't find the pronounciation of the Tetragrammaton in the Septuagint since the Septuagint is written in Greek. The Tetragrammaton is written in Hebrew.