If a sefer Torah was written in Greek, would it have been publicly read from in the Greek translation only, or would they also use a Hebrew scroll?
The Mishnah in Tractate Megillah, Chapter 4, discusses the laws concerning "Holy Scrolls" which are read during Shabbat and the holidays. It is presumed that the "Holy Scrolls" were written in Hebrew. This tradition of reading the Torah in Hebrew likely goes back to the prophet Ezra, and may extend even further than this. So it is unlikely that a Jewish community would ever have substituted a Greek Torah scroll for a Hebrew one in a public reading.
As to why a Jewish community would have a Greek sefer Torah in the first place, we need to consider the historical significance of the Septuagint. The Septuagint (sometimes abbreviated as the LXX) was a Greek translation of the Torah believed to have been written in the 2nd century BCE. For Jewish communities whose first language was Greek, such as Alexandria and possibly Qumran, the Septuagint was a valuable and revered book at that time. From the 2nd century BCE until roughly the 2nd century CE, Jews living in these communities used the Septuagint for study and reference. Scrolls were the norm in those days, so if a Jewish community wanted to have a Greek sefer Torah, such as the Septuagint, they would have a Greek scroll.