Hello,
1) Yes, it should be possible. The button is a part of the slide content so it depends on your settings there. Please describe the desired display of the slider/content in detail and provide temporary wp-admin access, so I can edit your Home page.
2) [8theme] Slider Settings > Arrows background color > http://prntscr.com/pfjf0p
3) Try to upload an image in this size http://prntscr.com/pfjh1d
Regards