I can't find the place to hold an event with sandra even though I have great affection with her (135) and I can't solve the puzzle in the skylo tower, I've found almost all the buttons, being the ones on the chair, statue, mirror, on the bookshelf and in a pile of stones (at least I think those are stones of another color ) but the last button I never think no matter how much I look

note: it's good to say that I also expelled a sorcerer who was making skeleton monsters

I can't say for certain what you're missing, without knowing what step of her events you're at, but the walkthrough should be able to solve both questions. :)