Hi Simon,
For question 1:
You can find the design files by click the gear icon > Design Manager > Edit Master Page/Display Templates/Page Layouts. To confirm which master page file you are using, please go to site settings > master page. Then go back to SharePoint designer and search for the file name to locate the place of it.
For further questions about customization, we’d suggest you post a thread in our MSDN forum which is a specific channel for handling such issues.
For question 2:
The screenshot seems to show the button for adding a new page, and from the thread, it looks like you want to add a new site. Can you double confirm which one are you looking for? Add a page or add a site?
Regards,
Ryan