Need help with template_preprocess_node
Blocks are rendered in hook_preprocess_page. At that time, all implementations of hook_preprocess_node have already been executed, so you cannot influence how the node is displayed (apart from messing with the HTML in $vars['content']).

If you use Drupal's Form API to implement the select list and the submit button, Drupal will redirect you after submitting the form, so you do not need to reload the page manually. See Post/Redirect/Get for why it is generally a good idea to redirect after a HTTP Post.