Why is 'Sabbath' often plural in the Greek text (both LXX and NT) yet translated as if it were singular? In my research online, I've been finding that this issue is hotly debated between Seventh Day Adventist scholars, Nestorian scholars, and those belonging to other Christian traditions (due to the former's 'theological stake' in the issue). I am looking for non-doctrinal analysis of the text only (as far as possible). I am aware that an underlying argument is whether or not relevant portions of the New Testament were translated from Aramaic, but for this question I would like to focus solely on the Greek text and other Greek literature that might shed light on this question.
I am also wondering if the use of 'Sabbath' to refer to 'week' is novel to Christian literature, or if it used like this elsewhere (I am aware of the use in the Didache, but this is Christian literature).
The backdrop to this question is these other questions, if you'd like some concrete examples: