For the bolded part (adding cha to wis based checks) does it have to be the actual night sky or can you use an item that shows the sky? (i.e a picture, drawing, viewing globe, illusion spell of the night sky)
Guiding Star (Su): Whenever you can see the open sky at night, you can determine your precise location. When the night sky is visible to you, you may also add your Charisma modifier to your Wisdom modifier on all Wisdom-based checks. In addition, once per night while outdoors, you can cast one spell as if it were modified by the Empower Spell, Extend Spell, Silent Spell, or Still Spell feat without increasing the spell’s casting time or level.
Also, it doesn't say wisdom based skills checks, so would it work for other wisdom based checks (will saves, spells/SLA that are based on wis, etc)
I don't know about the rules portion of wisdom based checks. But you would have to for sure be able to see the actual sky. You are looking for a guiding star to help you. Not a picture of a star.