You're still equating 'elohim' with 'G-d' and that's where your stance breaks down. Hebrews didn't ONLY apply that term to YHWH, but to ALL 'spiritual' beings; angels (Gen 32:1-2), demons (Deut. 32:17), deceased human dead (1 Sam. 28:13), the 'gods' of YHWH's Council (Psa. 82:1), and the 'gods' of the other Nations (Deut. 32:8-9, 43). So, when I state that Jews recognized other 'divine beings' (elohim) but the only elohim they worshipped was YHWH, that is completely consistent with Jewish monotheism. I'm not saying they worshipped these other elohim, just that they believed they existed.
I also posted a book that shows what Jews believed PRIOR to the onset of Christianity, that being 'binitarianism' which wasn't declared 'heretical until the 2nd Century because of Christianity. The author was a Jew and believed it to be heretical, but showed that Jews believed this to be orthodox for over 1500 years and did not violate the shema. It actually affirmed it.
Christians didn't corrupt the faith. The text of the OT (and intertestamental literature) forced them to acknowledge the fact of YHWH's uniqueness in 'divine plurality'. Jews, themselves, contemplated the 'second YHWH' in the personages of Moses, Jacob, Enoch,Wisdom, and other Jewish historical figures. When Jewish Christians posited Jesus as that second 'person', that's when they split.
It'd take me a while to post all the references that show the trajectory of Jews towards 'Trinitarian' belief in YHWH, but ancient Jews already had the categories for it within their belief system. For instance, read Amos 4:11 and Gen. 19:24.