Thank you for posting in a very detailed post information.
1.) Please make your slider along with the layers as responsive as it can be. You can check out this video tutorial on how you can create an auto responsive slider:
2.) Since you want to add the slider below the navbar, please remove it from your first section. To know more how you can assign the slider below the navbar, please check this out:
3.) For your poster images and text, please edit the page and click each column. You will have to align it to the center.
4.) The image behind the slider will be removed if you do the solution in #1 and #2.
Hope this helps.