If you look at the Hebrew I think you'll find your hearing is really good. When I can't make sense of what I'm hearing I try to write the sounds in Hebrew and then break them up into words. It only really works if you can remember the vocabulary but it does help. My first attempt however was nonsense, I didn't hear the syllables as clearly as you!
Well, as the verb חָזַר widely replaced שָׁב in the meaning to return, you can use חֲזָרָה for returning, but the bare infinitive, used as a noun, שׁוּב is also used. Bare תְּשׁוּבָה has reverted in general use totally to answer, as probably already used in Hi 21.34 and 34.36. Even Biblical תְּשׁוּבַת־הַשָּׁנָה turn of the year seems to be obsolete now. The Middle Hebrew meaning repentance by the way is nowadays disambiguated using the phrase חֲזָרָה בִּתְשׂוּבָה.
I suppose that ideally it should be schwa. (Well, ancient Hebrew /invented/ the word שווא (-: ). However, very few native Hebrew speakers, even if they do their best at formal pronunciation, would actually have more than one level of /e/. Maybe old newcomers whose mother tongue was Arabic and they retained traditional reading of the scriptures. But I'm no expert on this.