According to this page (paragraph "Learn from scroll") a witch can teach her familiar new spells from a scroll. Are these spells restricted to the witch spell list, or can it be any other spell as well? If so, what are the requirements for the witch to a) learn the spell and also b) to use the spell?
Unfortunately, only witch spells
That's a subsection under the heading Adding Spells to a Witch’s Familiar, which states
A witch can only add spells to her familiar if those spells belong to the witch’s spell list.
The typical witch's familiar only holds witch spells, even if those spells are from a scroll of non-witch spells that the witch has burned, turned into a tea, and fed to her familiar.