There is a german wikipedia article with pictures.
https://de.wikipedia.org/wiki/Kienspan
Usually they used spruce wood because it does not create soot.
For example, there is are excavations around https://de.wikipedia.org/wiki/Keltenwelt_am_Glauberg and AFAIK it's still not clear if that area is mostly a castle, or if it was mostly some cultic thing -> nothing written, not even in clay or chiseled into stone.
Similarly, in this article https://de.wikipedia.org/wiki/Hallstatt_(Arch%C3%A4ologie) that describes the salt mining in Hallstadt since the bronze age there is only so much that they know.
So, did they use torches? They have decayed over this time.
We know however that they had some kind of meat industry at Hallstein. They had plenty of salt, and so they used that to make meat durable. Animal skulls survived the times ...
That is a common misconception. We don't have much in the way of ancient Celtic writing, or even enough, but we do have some . The best known example is the Coligny Calendar. Some of the Celtic peoples used the Greek script and variants. There is also debate about how old Ogham is (although AFAIK there is no evidence of it from Hallstatt and La Tene).
I would not be surprised if some pre-Roman Celtic writing does turn up from this region at some point.