Giving her some natural resistance to overheating and making her get insanity from rain sound good for me.
SHE REALLY HATES RAIN.
Immunity to fire she can do without in my opinion. She must keep a degree of sanity regen near fire though. The whole freezing when insane is just nonsense. It has to go.
Honestly the lighter should probably be replaced by her having mastery over all things fire related.
For example lighting the firepit/endo firepit more efficiently (more for less) or maybe making special torches that have a higher light radius and durability? That way she could give improved torches to her friends. She could also start with the one of those improved torches in her inventory so she can remain newb friendly. Perhaps also making better campfires/endothermic fires on the go (bigger temp radious and/or duration.)
Basically if there's a willow on a server you should want her making the fire related things, it's her expertise.
She could also be a niche character for winter/summer starts with the aforementioned traits.