Thanks for writing in!
1.) Could you please clarify a little further of which part of your header you want to reduce the margins? Kindly send us some screenshots or illustration so that we will have a clear understanding of what you want to do.
2.) We could not log in to your site because the given password is incorrect. Meanwhile, please check out the font manager from this articles: https://theme.co/apex/forum/t/font-manager/101/1
Please be advised that most of the X theme settings were already remove in the customizer. You can access the theme options and the typography in X > Theme Options or X > Theme Options > Typography
3.) Your slider is already fullwidth. Do you mean fullscreen? Please edit your slider and change the slider layout in the slider settings. You will also need to edit the page and disable the slider. You will need to re assign in above the masthead. Please check out the Slider Settings: Above the Masthead in this article: https://theme.co/apex/forum/t/features-sliders/97/1
Hope this helps.