How to have one page show posts reverse-chronologically, but the others chronologically?

Question

You want a filter on pre_get_posts restricted to the home page. Something like:

function pregp_wpse_214875($qry) {
  if (is_home() && $qry->is_main_query()) {
    $qry->set('order','ASC');
  }
}
add_action('pre_get_posts','pregp_wpse_214875');

You may need something other than is_home(), depending on how your “home page” is set up. The other most likely option would be is_front_page()