Drush downloading to an old version of Drupal
Drush is path-sensitive and can be installed system-wide; no need to install a separate instance.

To run Drush commands in the context of a site, you need to be in the web root of that site.

cd /path/to/mamp/drupal_root;
drush cc all; // Clears cache for the current site only


If you run commands outside of a Drupal root, the effects will vary. Modules will be downloaded to the current folder, cache-clear will clear Drush's internal cache, and some commands simply won't work as they don't make sense without a Drupal boostrap.