How to reinstate previously saved option?

The only way WordPress is going to know what the previous option was, is if you save it before you change it – since you’re replacing the built-in option, it has no way of knowing how to get back to that.

So, you could add a custom ‘backup’ option that you restore on deactivation.

Something like this when your option is first set:

function set_hp(){

   update_option( 'wpse_231804_backup_page_on_front', get_option( 'page_on_front' ) );
   update_option( 'wpse_231804_backup_show_on_front', get_option( 'show_on_front' ) );

   // ... include the rest of your code here ...

}

With a simple reverse on deactivation:

function myplugin_deactivate(){

   update_option( 'page_on_front', get_option( 'wpse_231804_backup_page_on_front' ) );
   update_option( 'show_on_front', get_option( 'wpse_231804_backup_show_on_front' ) );

}