#1. It was forbidden for Jews to marry non-Jews in the Russian empire, so I don't see how your great-grandfather could have married a Tatar woman, unless he converted to Islam. There was a not insignificant number of Jews who married Christians, but only after they converted to Christianity.
Have you considered taking a DNA test - it would show an East-Eurasian marker. I believe FTDNA and likely other tests from the main companies should be able to detect it. MyHeritage is notorious for showing small percentages of highly-speculative results, so I would avoid them.
#2. Regarding Ekaterinoslav. By the time it was occupied by German troops it had been known as Dnepropetrovsk for 15 years. Do you have an estimate on birth years of the folks you described? It might be helpful to try to write down an estimate and then search Yad-Vashem site, based on the names and dates.
You flagged this post with Belarus hashtag, is that related to item 1?