Safe to delete cache_form table while site is in maintenance mode?
First off, cache_form is not really a cache table, but a temporary table used to contain data necessary to verify the form being submitted to Drupal is not forged.
If you were to empty that table, users would encounter validation errors for the data they submitted from the form they were using. Since the site is in maintenance mode, that should impact the only users who are allowed to access the site when it is in maintenance mode, which includes you. You just reload the page containing the form, and fill it again; it's not a permanent error.