Pressflow caching directives
The varnish module handles clearing the cache for you when a node is update via a socket connection to varnish. It can also be configure to clear the cache based on a lifetime period. If you still need to clear the cache based on your own custom requirements the module also provides a helper function such as _varnish_terminal_run that allows you to send commands to varnish from your php code.

Also in regard to the bullet point checkout the drupal_page_cache_header_external in pressflows bootstrap.inc file

There is also more documentation of varnish with pressflow over at fourkitchens wiki