I've never had trouble staying on the trail from cold springs in the dark. The trail itself is pretty clear, and where it is covered in snow it should have a serious boot track by now.
There is a left turn you can make on the trail that is incorrect that can trip people up a bit, but familiarize yourself with the map and route and pay attention to the terrain versus what you expect and you should be fine.
As for helmet, most people I've seen don't take one for the south side (which is what I assume you're doing). I've taken mine on the south side before though, so it is a matter of personal preference. There really aren't any terrain features that worry me about rock/ice fall, but I was always taught to pair a helmet with crampons. YMMV.