Placement of the button

I think the best way is to provide the back to top as a block so you can place it, but thats not possible right now. But it should also be possible to change the code in js/back_to_top.js so the selector body instead is your kos-content div, i e:

  var exist= jQuery('#backtotop').length;
    if(exist == 0) {
      $(".kos-content", context).once(function() {

I cannot build patches where I'm at work, but here is what I just did to Backtotop 7.x 1.4 to get this functionality mentioned by acke on the Back-to-Top-settings page. Note: I put all code inside php-tags just for better readability.

However, you'll have to alter your CSS also: unset top and left of the button-css and prefer to work with margins. There is some guidance here: http://stackoverflow.com/questions/6794000/fixed-position-but-relative-to-container/15559540#15559540

Maybe acke can add this to his wonderful module.

back_to_top.admin.inc (changes from line 59)

    ),
    '#default_value' => variable_get('back_to_top_button_place', 1),
  );
  $form['back_to_top_parent_container'] = array(
    '#type' => 'textfield',
    '#title' => 'Parent Container',
    '#description' => 'Change the Parent Container of the Button, e.g. for wide screens. Default is "body".',
    '#default_value' => variable_get('back_to_top_parent_container', "body"),
    '#size' => 30,
    '#maxlength' => 60,
  );
  $form['back_to_top_tag_or_div'] = array(
    '#type' => 'radios',
    '#title' => 'Is the parent a tag or a div?',
    '#description' => 'In case of div make sure to provide a unique ID above and not a CSS-class. Default is "Tag".',
    '#options' => array(
    'tag' => t('Tag'),
    'div' => t('Div')
    ),
    '#default_value' => variable_get('back_to_top_tag_or_div', 'tag'),
  );
  $form['back_to_top_button_text'] = array(
    '#type' => 'textfield',
    '#title' => 'Button text', 

back_to_top.module (changes from line 37)

function back_to_top_page_build() {
  $settings = array(
    'back_to_top_prevent_on_mobile' => variable_get('back_to_top_prevent_on_mobile', TRUE),
    'back_to_top_prevent_in_admin' => variable_get('back_to_top_prevent_in_admin', TRUE),
    'back_to_top_button_type' => variable_get('back_to_top_button_type', 'image'),
    'back_to_top_parent_container' => variable_get('back_to_top_parent_container', 'body'),
    'back_to_top_tag_or_div' => variable_get('back_to_top_tag_or_div', 'tag'),
    'back_to_top_button_text' => variable_get('back_to_top_button_text', 'Back to top'),
  );

js/back_to_top.js (changes from line 6)

(function ($) {
	Drupal.behaviors.backtotop = {
		attach: function(context) {
			var exist= jQuery('#backtotop').length;
      if(exist == 0) {
		var $parent_container = Drupal.t(Drupal.settings.back_to_top.back_to_top_parent_container);
		var $tag_or_div = Drupal.t(Drupal.settings.back_to_top.back_to_top_tag_or_div);
		if ($tag_or_div == 'div'){
			$parent_container = '#' + $parent_container;
		}
        $($parent_container, context).once(function() {
          $(this).append("<div id='backtotop'>"+Drupal.t(Drupal.settings.back_to_top.back_to_top_button_text)+"</div>");
        });
      }