Some great oldschool wisdom so far here on a tough problem . my question/suggestion is water pump related , I have over the years encountered the following problems (on assorted makes/models) with new pumps ; impellers that are different diameters , different depths , vanes rotating opposite directions . Not always obvious at a glance and will usually circulate water just not very efficiently making diagnosis challenging ....... good luck , hopefully an easy fix is found soon allowing you more time to make the rest of us look bad with your next project !