The main challenges to climb the lower sections of the MR in the dark are route finding (the MR is not an obvious trail like the main route) and the Ebersbacher ledges. If you scout out that section in the daylight, you should be able to find your way up to the E ledges with a good light. Climbing the E ledges in the dark can be done if you know the way and are comfortable with the exposure. Again, you will want a good light. After that, you can get up to lower and upper Boy Scout lakes, but study the trail descriptions on this board and in Doug's book.
On the way down, your main challenge would be getting to the E ledges in the dark. If you are doing this as a single day hike, consider going down the main route if you think you will be in the dark near the bottom.
As for snow/ice, it would not be unusual to have some snow days in the early fall. Maybe not enough to accumulate, but keep an eye on it. In general, the fall weather is the best all year, but every day on Whitney has its own weather.
