Can I set my WP_Query to be a Main Query?

This is an overly complicated approach. pre_get_posts will work for any query and there are numerous ways to control when/how the callback behaves:

1. Add and remove the callback:

   function dummy_action($q) {
   
   }
   
   add_action('pre_get_posts','dummy_action');
   $q = new WP_Query(
     array(
       'post_type' => 'post'
     )
   );
   remove_action('pre_get_posts','dummy_action');
   

2. Use a self-removing callback:

   function dummy_action($q) {
     remove_action('pre_get_posts','dummy_action');
   }
   
   add_action('pre_get_posts','dummy_action');
   $q = new WP_Query(
     array(
       'post_type' => 'post'
     )
   );
   

3. Feed the query a parameter you can use as a trigger:

   function dummy_action($q) {
     if ($q->get('action_trigger')) {
       // do something
     }
   }
   
   add_action('pre_get_posts','dummy_action');
   $q = new WP_Query(
     array(
       'post_type' => 'post',
       'action_trigger' => true
     )
   );
   

You do not want to “assign” a secondary query as the main query. That will cause the same issues as caused by using query_posts(), which essentially does assign a secondary query as the main query.