Hey Yuko,
I will cover the issues in the same order:
1). As I mentioned earlier, the “One-page” template should only be used for the “container” page, i.e. once per website. All other pages that will comprise the sections of the single page layout should have any other template declared.
2). Please try downloading this file and use it as the source when importing the slider.
3). If you cannot see the changes you’re making, it is most probable that you’re using a caching plugin, e.g. WP Super Cache or similar. If you are, please clear the cache and deactivate it until you finish developing; if there are no third-party plugins installed on your WP instance, please let me know, I will look further.
Sincerely,
Andrii / Satori Studio